Caliphates in 300 AD, before Islam exists?
Technically, if the whole area is conquered by Rome or Presia, that counts. The penisula is not only united, but also part of a bigger empire.
But then the issue is how?
Pre-Islamic Arabia was as diverse than, let's say pre-roman Gaul : diverse tribes not always sharing same origins or exact languages, influence of foreign countries competing (Ethiopia, Rome, Persia, Egypt, etc.)
While benefiting on being on the major trade road to India, it certainly favoured both great focus on coast at the expense of desert seen...well as pretty useless and dangerous and to powerful neighbours intervention that had little interest to see Arabia being united.
Depending how we interpret the "united" it can go to feasible to really unlikely.
If only the coast count, then Persian empires, probably Alexander and maybe Rome (but relativly unlikely as it would force them to stretch their troops to importantly) could do.
An unfication made by Arabs alone seems really hard without inner and external incitatives (aka unifing factors and decline or even asence of foreign intervention).
An unification including desert seems really really hard to do, never having being really made including in caliphal times.
The very early caliphs seem to have managed, to a point. It didn't went far into the Umayyad era I think, if at all.
It's really debatable they managed to get much more than a nominal acceptence. Nor that they cared that much : why care about bitching Beduins when you can get rich provinces from Byzzies and Persians?
It's quite intereting to see that the two more important factions in Umayyad period are issued from the western margins of Arabia.
It's vague, of course, but the Apostasy Wars can show that Nejd tribes tried to reject Caliphal domination - but not Islam - as far than Muhammad's death. Later, Bedouin recieved stipends for allowance relativly irregularly as not playing a really dyamic role.
You had another documentated revolt in the late VII century (Ibn al-Zubaïr) showing that even in the western shores (while having being favoured by the first Umayyads), Arabs weren't really united.
When you say "to a point" I agree, as long we're talking of a more or less vague point(mostly because of lack of documentation : I wonder if it's a study about it)
Challenge: have the Arabian Peninsula unite around 300 AD.
What happens?
What was the climate in Arabia like at the time? Was it as dry as today, wetter or dryer? That would have a major impact on the ability of any army to conquer/unify it. I know there used to be civilization in the empty quarter but can't recollect the era it existed.