Two sets of facts for different purposes. Just like how we know that the stars are only a few miles away, but for the purpose of science they are millions of miles away. (/minitrue)
The sinking of your habitat can do that to you, otherwise it has a good evolutionary niche.
Also, the flying ancestors are still around, so if anything happens they can come back in another 20 000 years.
It went extinct because the sea levels rose and the island it inhabits was entirely under water. Honestly, we're headed that direction - they might be in trouble again in relatively quick order.
It evolved to be flightless because it was useless on an island with no predators, it drowned when the sea levels rose and covered the island, its closest relative (from whose ancestor it had evolved) flew back to the island once the sea levels fell, it evolved to be flightless because it was useless on an island with no predators.
"Aldabra went under the sea and everything was gone," Julian Hume, paleontologist and author of the study, said in a press release from the Natural History Museum in London. "There was an almost complete turn over in the fauna. Everything … went extinct. Yet as the Aldabra rail still lives on today, something must have happened for it to have returned."
Even if they could float, and reach the coast, they'd have immediately gone extinct due to not being adapted to having predators and being outcompeted by their flying relatives.