The expression "back to baseline" comes from Science and Engineering and literally means that something has gone back to the previous average flat level (for example: "the power line noise level spiked when your turned the machine on but is now back to baseline")

Edit: not average, but actually specifically the original flat level below which things would not fall. Sorry, it's kinda hard to explain in words but very easy to point out in a graph or a scope were it's just this flat line to which things always return.

That expression makes sense if you're talking about the rate of growth itself (i.e. the Lemmy rate of growth spiked at the time of the Reddit changes and eventually went back to baseline, since Lemmy is not growing any faster now than before the Reddit changes) but it doesn't make sense if you're talking about user numbers since the number of Lemmy users grew a lot with the Reddit changes and never went back to the average before them, not even close.

Your original post is not clear on which of those things you're talking about when you wrote "back to baseline" and your subsequent posts are mainly talking about user numbers, giving the idea that that's what your "back to baseline" is refering to, in which case you're using that expression incorrectly.