The "reddit mod" the comment you're responding to would be one in charge of a particular subreddit, not of the entire network (the latter would be an "admin" in reddit parlance).
A quick perusal of https://www.reddit.com/r/SubredditDrama/ should be sufficient to understand that subreddit mods indeed do go on power trips of capricious banning, including for the grievous sin of mentioning a competing subreddit.
Many of the popular subs will retroactively ban users for posting in other subs.
I was banned from a fair amount of subs because I had a trivial argument with a mod in one sub. IIRC it was literally an "argument" about humidity being worse than dry heat, lol.
Seeing this repeated on twitter, even if its for a day, is alarming.
I remember there was one point when Facebook was blocking links to minds.com.
However, that was quickly explained away as automated spam filter false positive when it was publicized. So even if it happened before, I can't think of any case other than Twitter right now when it was so open and explicit.
A quick perusal of https://www.reddit.com/r/SubredditDrama/ should be sufficient to understand that subreddit mods indeed do go on power trips of capricious banning, including for the grievous sin of mentioning a competing subreddit.