I kind of feel like Kirani James is clean and those other two are not
That Michael Johnson's stride frequency tough...
The guy would barely lose anything in the last 100m.
I also think I saw some race in which he ran sub 44 with the first 200 slower than the second.
The thing with the 400 is that to be "on pace" for a time you actually have to seem to be ahead of pace by the 300 mark. It might look like some runners speed up at the end, but everyone is slowing down. Negative splits in the 400 at this level are extremely rare. It's just not how the race is ran.