you misunderstood the OP, he is talking about manipulation via difficulty adjustment , but that won't work because the timestamps are validated.
I don't think I misunderstood the OP. The argument is that even if timestamps were not validated, it
still wouldn't work, because the fake chain won't be accepted as the longest. So there are at least two reasons why this is not an actual problem.
If timestamps were not validated, I think it actually would be a problem because attacker could build a very long chain very quickly and broadcast that whole chain.