Seems like a lot of specifics were left out of the article/fluffed up press release in favor of vague approximations, like thousands of kilometers. The Perth Abyssal Plain at its widest is only just of over a thousand km from East to west and as it lies just off the coast of Western Australia these islands couldn't be 'thousands' of km into the middle of the Indian Ocean. They sound a bit like the Orphan Knoll just north of the Grand Banks, a small piece of continental shelf left behind when two continents rifted apart. But it was so small that it kind of slumped down and submerged as rifting pulled it away from the continental margins.
As for these 'sunken islands' I assume they are known features which are now being surveyed in greater detail and found to be continental fragments rather than volcanic seamounts or oceanic ridges or something else. Since we've had reasonably accurate bathymetry for quite some time now it seems unlikely that this could be an previously undetected prominent rise in the middle of an abyssal plain. Especially given the relative size of the Perth Abyssal plain compared to a Tasmania sized feature. I assume they will be used to boost Australia's EEZ under UNCLOS provision for extensions of the continental shelf, perhaps that's where the funding for this expedition came from.
It's too bad they didn't specify the location but I'd guess based on the description that it was at the northern or western fringe of the plain. Perhaps they were actually talking about the Wallaby Plateau? Though I'd assume if that was the case they'd refer to it by name. And of course its assumed to be volcanic in nature like the Broken Ridge-Kerguelen group. But Australia has been doing surveys there in recent years to advance claims under UNCLOS and a serious revision from volcanic to continental origins would definitely change the understanding of the formation of that part of the Indian Ocean.
The Dirk Hartog Ridge seems unlikely given its formation and the Zenith Plateau, Batavia Knoll and Gulden Draak Knoll don't technically border the Abyssal Plain, at least I don't believe so. It will probably be cleared up when this is published in an academic journal and the full scope over their findings are analyzed.