I think there's a not-insignificant number of people who have been banned for cheating in the past that play now (at top level) and nobody thinks they're cheating. Most notable cases off the top of my head for 6s would be kev & nyxi (banned in UGC). I believe yight/circa as well was strongly believed to have cheated at one point because of their proximity to bullet/roxanne/etc, but was never banned because ESEA.
There's also other stuff such as being banned for spy scripts (mostly highlander players, some have played 6s though) in league matches, and of course instances of people cheating in pubs (habib).
Notably though, all the people above were not invite once unbanned, and so I imagine their rise was more believable. I imagine if somebody was unbanned and then immediately played invite, people would complain more.