Interestingly, when you look at e.g. thereyougo's submission history https://news.ycombinator.com/submitted?id=thereyougo there are [dead] but not [flagged] or [dupe] posts from towardsdatascience, imperva, and testcraft. Maybe the sustained campaign got those companies' domains banned from HN? Or there was something specific to those submissions that triggered the spam filter.
Yes, what you described is what happened. That's also why the submissions appear to have stopped about 25 days ago, even though the submitter was trying. The recent history will contain more "cover" (legitimate) articles than sockpuppet posts because the recent sockpuppet ones never saw the light of day.
