Over here, new users who add a marginally relevant post to an old thread are typically link spammers. They aim to drop links to commercial websites, in an attempt to improve those sites' search engine rank.
They search for threads which discuss a certain topic (relevant to the site they are trying to push), create a new account, and post a message that's low in content but innocent. Then they come back a few days or a week later, when the thread has faded away again, and edit their post to add a spam link. I am pretty sure that these actors are not AI-based, but low-paid crowd-sourced humans.
Your Putin friend may be a different story, but hard to assess since you did not provide links. I assume those posts have been removed in the meantime?