I believe the critical aspect here is not the specific cheating that chess.com found, but that this appears to contain written statements by Niemann himself which contradict his public statement.
And if it now turns out that he lied in his confession, too, then that's a really bad look w.r.t. his trustworthiness.
- "written statements by Niemann himself which contradict his public statement"
The article doesn't say that Niemann's admissions to Chess.com were about cheating in prize-money tournaments, nor the other disputed facts. The spreadsheet of incidents they show us isn't what Niemann admitted to cheating in, but was Chess.com's internal anticheat flagged -- we can tell because they label it as "suspect games" and it uses qualifiers like "likely". The inferences that the cheating was for real money prizes, or at an age older than 16, or on for-profit Twitch streams, are drawn from from this list of suspected games.
We don't currently know what facts Niemann confessed too: it's not public whether the facts Niemann is allegedly lying about overlap with the facts Niemann admitted to in writing in 2020. WSJ may have evidence that's dispositive on this point (i.e. those Slack texts), but they haven't printed it yet.
Confessions have an important role in forgiveness, but given that, the public arena is never the right place for confessions; i.e., do you expect someone to say "Actually, I also hit on another employee last May." That would get you nailed to a cross and no lawyer would ever advise that except on the calculus of further damage control.
And if it now turns out that he lied in his confession, too, then that's a really bad look w.r.t. his trustworthiness.