Yes, him having 666k followers is a fucking coincidence. A month ago he had less and a month from now he'll likely have more. Everyone with over 666 followers has at some point had 666 followers. If they surpass 666k followers they'll have had the number twice (or 3 times if you count 66.6k in between), so on and so forth. It means quite literally nothing. I've numerous times seen YouTube likes or Reddit upvote counts be at 666 or upvoted/liked something myself only to realize that I was the 666th. I've also upvoted or liked posts on 665k (same for 66.5k) wondering if I'd be the one that pushes it up by one. A few times it did, I thought "ooh spooky", and then moved on as it means nothing.

Now if Joe Biden's Twitter account magically goes to 666 million (example, could be 666k, 666, etc.) followers and just sits there for months on end then you may have something, but until then it means nothing as thousands or even millions of people have had 666 as their follow count at some time or another (including Trump and many, many, many other patriots we trust).