I don’t know where that comment (screencapped memechat) was posted. It would be too burdensome to find and it’s not strictly necessary to satisfy your concerns since there is other evidence that’s conclusive.
imgflip.com/i/5gir2s#com13066150
Check that comment, which IG linked below. This comment is RichardChill commenting (using all sorts of RichardChill-specific tics like emojis and “cope”) as Abdul-Kareem in protest of his ban.
How could this person have had access to a Memechat between RichardChill and a global mod unless he was actually RichardChill? Yes, in theory RichardChill could have screencapped that and then passed it along to a friend of his but now we’re really getting into the weeds.
You mention the concept of fabricated screenshots. Well, in this unique case you can actually ask Ser himself whether the memechat was fabricated if you still have concerns.
The mods of this stream made a determination this account is RichardChill and the reasoning is significantly above speculation and conjecture.
If you disagree and have solid evidence it’s not Richard, the proper way to challenge a user ban passed by Congress is through a formal impeachment proceeding or Congressional motion.