You mean the supposed "nephew" or something four times removed that had to basically win by conquest? Herman Ooms indicates there's doubt whether he wasn't just some random guy they proclaimed a descendant.
I think it would be harder to whitewash a Soga take-over than Keitai. Though no doubt they'd try.
I'm not sure who "they" are in this scenario. I'm not sure if Keitai won by conquest, or if he was the one who claimed descent from Ojin, but my main point is that a change in dynasty might have occurred, so the Soga wouldn't be the first to do so.
Also, the Kojiki and Nihon Shoki weren't written yet, so a Soga takeover could be justified. I should point out that the accounts of Buretsu seem fabricated after the accounts of the last kings of Xia and Shang in one of the Chinese texts, so the idea of a dynastic Mandate of Heaven does seem possible if the Soga claim to have it.