I am not sure that one would claim that content creators are there rather than people copying content from various porn and making it accessible.

Do the graphs on that page suggest a substantial increase in the rate of content being submitted to Lemmy instances in the past year?

Memes and jokes are fine... if that's what you are after. I'll note that my /r/all page is devoid of most of them as the reddit subs that I'm on tend to not be as meme saturated.

If you are after deeper discussions of programming topics or hobbies, Lemmy doesn't seem to show the sustained growth or content being added that provides for interesting reading. https://startrek.website is probably the exception to this as they've maintained a rather focused set of communities and so it is easier to find content that isn't memes.

I haven't been able to find an equivalent of /r/DIY, /r/boardgames, /r/soloboardgaming, /r/books, /r/factorio (and its set of subs) on Lemmy.

If one wants to claim that the content creators of /r/memes have mostly left and gone to Lemmy... ok. I'm not as familiar with that subculture. If you want to say that the content creators of /r/books have left and gone to Lemmy - I have seen no indication of that happening.