Yes but note the specific details of that assumption and their reasoning: it’s based on reddit’s announcement of the security incident a few months ago which starts:

Based on our investigation so far, Reddit user passwords and accounts are safe…

Now, look again at what BlackCat has promised in this leak:

Instead, BlackCat is teasing such revelations as “all the statistics they track about their users,” and data concerning how Reddit “silently censors users.”

80 GB of “statistics and data” about Reddit’s users is a lot. It may not contain raw IP addresses, but we know that IP matching is one of the ways Reddit catches sock puppets, so there may at least be a hash that could be used to identify accounts held by the same users.

Am I going too far worrying about PMs and other details? Maybe. It really depends on the honesty and competence of BlackCat and Reddit, and the article author’s assumptions based on their statements.