I wonder if these are an alternative to IndieBlocks or something else. Can you make templates with the correct microformats in the Site Editor?

I used to use hand-typed (or semi-automatically generated) HTML before moving to the block editor (and IndieBlocks), so, yes, a somewhat more hand-rolled solution can be such an alternative.

In fact, what blocks do is exactly that; they take certain “attributes” and generate HTML off of them, using a “template” of sorts, and inject the result into WordPress’ database. That’s also why your post content wouldn’t break, if you were to disable IndieBlocks. (I.e., while the dynamically generated “theme blocks,” like the Facepile block for webmentions, would “disappear,” posts should remain intact.)

You can make “block templates” for the block editor—don’t think you’d wanna use the site editor, which is more like a theme editor. Like, you could probably create a “base” template with, e.g., a Custom HTML block in it, ready with microformatted HTML and dummy text. But then you might as well use a more user-friendly block (I think).