And sometimes it needs to be thrown out, if it can't be upgraded efficiently. It might not even be bad, just programming with a smaller scope in mind and having bit-rot through too many changes from too many different programmers. In western societies that is called "too many cooks spoil the broth"
Faatal is not a russian, so his answer doesn't conform to russian standards. Also he was asked this and it is also polite to answer truthfully, and that is something people here in the forum probably value.
More importantly though, it seems he was hired in 2018, but A15 was released in 2016, A16 in 2017. And I think he was specifically hired for the AI rewrite. Probably because nobody on the team wanted to touch the AI code and if you are right they even burned their fingers touching it.
In other words, we don't know who made the mistake, but (at least if my research is correct) it could not have been Faatal who made the bug. And in that case the achievement would stand.