I just re-watched the Kenyan DQ/ SA fall.
I think it was VERY generous. They Kenyan definitely moved out in front of the South African and they subsequently both went down. But it wasn't egregious. The Kenyan was clear in front, just not the full stride. They enforced this rule to the letter, which they rarely do. Plus the Kenyan wasn't cutting in, he was moving out. I don't think he was passing on the inside, then moving out. The best I can tell is that the Kenyan was being passed, didn't see the South African coming up on his outside when he moved out.
If you run up the back of someone and fall that doesn't warrant an automatic berth in the final in my opinion.
This is the rule from the IAAF competition rule book:
Any competing athlete who jostles or obstructs another athlete, so as to impede his progress, shall be liable to disqualification from that event. The Referee shall have the authority to order the race to be reheld excluding any disqualified athlete or, in the case of a preliminary round, to permit any athlete(s) seriously affected by jostling or
obstruction (other than any disqualified athlete), to compete in a subsequent round of the event. Normally, such an athlete should have completed the event with bona fide effort.