The user you linked made a single post and a single comment on the topic. You spread a ton of posts and comments across several subreddits.
I did cross-post it, and I think that is indeed what cross-posting is for.
The subreddit mods might have reported you to the admins as a spammer - that’s what they do when they see someone breaking site wide rules. Like it or not, they are under no obligation to communicate with you about it beforehand. It’s also possible that none of the subreddit mods had an issue with it and you happened to be noticed by an admin who saw you were breaking the rules.
If that's true, they would have also removed my post, even if a one sub's mod team missed it, I cross-posted it to a couple of other subs, did all of them just let my post grow and silently report to Reddit's admins? No, I don't think this is the case.
EDIT: As a long-time reddit moderator, I know what we would typically do if we saw spammers, we can report them as you said, but we also mainly (and first) remove their posts, but that did not happen across any of the subs I cross-posted to. And this does not explain what happened briefly with my r/LemmyMigration sub today morning either.