Not a minor stumbling block. A major one. The Amerindians managed without bovines and horses until the 15th centuries then got cleaned out by those who had them. As for a niche, just if they can reach the highlands. Slight problem, the rain forest. Humans can barely move through it without great effort. A open country creature like the horse that lives off grass is going to equal or greater trouble.
Seems you don't appreciate just how big New Guinea is, or the levels of ecological diversity on the island.
Yes, the central range sub-alpine grassland zone (turquoise) would indeed be the most suitable environment for horses. But for bovines? The water buffalo- or, rather, the swamp buffalo, as the species is known in South East Asia- resides primarily in freshwater swamp forest, and lowland rain forest. Now, look at the ecological map of New Guinea. What is most of the coastline?
Yes, if they can get to the coast. Same problem as above, the rain forest.
*****
For those are are not aware, it was not known by Europeans or Asians that there was an agricultural civilisation on the New Guinea highlands until the 1920s. That is because the rain forest is low in forageable food and and in order to travel through it travellers have to cut paths as they go. Europeans and Asians had been landing on the coast for hundreds of years so if either they had not penetrated the rain forest or if they had not come back there must be very good reasons. Reasons such as physical difficulty or if a few stranger turn up then top them in case they are a threat.
Which is the main reason why
Psophocarpus tetragonolobus, the proposed ideal founder crop for such a civilisation, is so ideal. It's native to the Sepik region, and it's been used by the Sepik peoples for centuries before the Europeans arrived. Here's a map of Papua New Guinea:
You see where the Sepik region is? Well, this crop is widespread, and grows wild across practically the whole of East Sepik- all the way up the Sepik River's drainage basin, and even along the coasts, from Wewak all the way to Awar in the adjoining Awar province. And here's a map of the Sepik river, which is navigable all the way up to Ambunti. Local villagers have lived along the river for many millennia, and the river has formed the basis for food, transport and culture.
The most populous grouping of the Sepik peoples is that of the Iatmul- taken from their name for single clans. The Iatmul are not a centralized tribe. They never act politically, socially, or economically as a single unit; villages are autonomous. People tend to self-identify not as Iatmul or, as they sometimes say, Iatmoi, but in terms of their clan, lineage, village, or sometimes just the colonial-era regional term, Sepik. IOTL, this was the first region of Papua New Guinea, on the west side of the island, which Malay traders routinely visited and interacted with, from the mid-15th century onward. And it was by way of this region that steel axe-heads were introduced at this time, traded into the Highlands from the coast. These developments saw huge population increases, and an increase in war, slave-trading and head-hunting.
In Iatmul legend, the original condition of the world was a primal sea. A wind stirred waves, and land surfaced. A large pit opened (according to the Iatmul, this primal pit is located near the Sawos-speaking village of Gaigarobi, in the middle of the map above), and out emerged the first generation of ancestral spirits and culture-heroes. The ancestors then embarked on a series of mythic-historic migrations. Where they trod, land appeared. Along these routes, the ancestors created the world through naming. Literally, they named all the features of the world into existence—trees, mountains, stars, winds, rains, tributaries, villages, actions, virtually everything in the world. These ancestral spirits' names are totemic, and they're claimed by specific patrilineal groups (clans, lineages, and branches)- seen as possessing divine essence, these form the basis for their religious system. Each clan's headed by a patriarch, who performs sacred rites to the clan's ancestral spirit to ensure the long-term welfare of the clan.
Now, compare this belief system, and this society, with that of Japan at the same stage in Honshu's path to civilisation, in the first 600 years after the introduction of iron. In the Yayoi period, as detailed previously, Japan was also a land of hundreds of scattered tribal communities, rife with inter-tribal warfare, with defensive settlements becoming increasingly common, along with both slave-trading and head-hunting (as evidenced by the headless human skeletons from this period which are routine archeological finds). Then, as in the Sepik region today, the Japanese, aka 'Wa' peoples, lived on fish and vegetables. And really, how is the Iatmul belief system any different from that of historical 'Kashinto'?
If the peoples of New Guinea been propelled into the Iron Age at the same time that the Filipinos had, and had the swamp buffaloes introduced at the same time, by the Sa Huyun or whoever else it was who introduced iron bloomeries and these animals across the Philippines- between 1000BC and 200AD- then the Sepik region would have been the first region where these new arrivals, settlers and traders via the Philippines, would have arrived and set foot in New Guinea. And this founder crop, this social set-up and way of life, this belief system- IMHO, it's just about the best tropical Japan analogue you could possibly hope for. And if it had been propelled into the Iron Age around 1800 years earlier, as Japan was? Well, why couldn't this region have given rise to a Japan-analogue civilization, with one clan emerging as the most powerful of all, establishing its own Imperial Court of clan patriarchs, and then going on over the course of several centuries to extend its dominion over the entire Sepik and Madang regions (making it roughly the same size as, or even larger than, the territory under the dominion of the Yamato court by the end of the Asuka period)?