I'm not sure why you're surprised, it's actually a relatively common occurrence, this, along with gravedigging old threads and replying to the starting post, instead of trying to keep up with the conversation in the last few pages. I locked two major topics of mine for this reason, people were either posting nonsense that had nothing to do with the topic itself, and other users were replying to the first spammer making a chain of spam posts that are irrelevant to the topic.

I wouldn't be surprised if the users you quoted were using AI or had copied their reply from somewhere else (plagiarism). People do a lot of dumb stuff to appear knowledgeable and show up in the first pages, making sure their reply isn't lost among many others.