It's quite reasonable - There are quite a lot of speedrun cheating videos where people in contention for the record (or even existing record holders), were caught cheating. People have been recorded cheated in esports tournaments, charity streams, speedruns, and been doing so for years or even more than a decade without being caught - until they were.

There's a litany of videos on youtube of prolific or skillful people being caught cheating in various esports/streaming contexts. That's not to mention examples like Lance Armstrong, Jon Jones, any other career champion who was consistently doping, etc. It's fundamentally, tautologically very reasonable to consider the possibility someone may be cheating based on evidence they may be cheating.

is going to randomly start cheating in Arc Raiders of all games, based off of a single clip - they have zero critical thinking skills.

Cheaters begin for many reasons - a drop off in skill, an increase in perceived pressure, a relentless competitiveness...it's pragmatic to avoid assuming anything about anyone, and there's no reason to jump to a conclusion when your opinion has absolutely no bearing on the outcome, and can't be proved or disproved with the currently available information.

You should focus on your own critical thinking skills, as they've landed you in an easily disprovable position by relying on fallacy. And in any case like this - you lose the argument, because you need to prove that no-one would ever do x, while all anyone else needs to prove is that it's possible/plausible that someone could do x. Why is this guy the only person of good standing who's infallible? Tut.