you just write in Markdown (the same thing Lemmy uses), pick a theme, and you’re good to go
That is far too basic for most websites. It's absurd to suggest that's a valid alternative for something like Wordpress + Elementor.
Squarespace should be on the table for static websites as well.
How so? It's not static that I'm aware of, unless you're exporting it to a file after using the UI to create it?
I assumed we were talking about direct replacements for WordPress, not hosted alternatives.
Well, as you said, Wordpress does a lot of things. Shopify, Wordpress, Squarespace, etc., are certainly interchangeable/competitors to a large degree. Wordpress has hosted options and is a default/main option for many hosting companies.
You can build a full website with every major function and design option with Wordpress. You can't with Jekyll and Hugo unless you can code.