I'm not sure I understand that assumption. He's been around the OSS community for decades (wrote some O'Reilly books about Perl, and wrote quite a few Perl modules, though that was more than a decade ago, and I dunno what he's been up to since). I do find the github a little sparse, but there is a website for SILE, which is better: http://sile-typesetter.org/what-is/

It looks like a personal project, rather than a "take over the world" attack on TeX. Seems like if it's an area that's interesting for someone, they might want to have a look. It does have some novel ideas over TeX, and is maybe more modern in some of its implementation details, even if it is nowhere near as complete or mature as LaTeX.

"Does it support mathematics?

Not yet. I understand that mathematics is important in scientific typesetting, but SILE is targeting a particular niche in the humanities. That said, it's not impossible, and it should not be difficult for someone to port MathJax from Javascript to Lua and implement mathematical typesetting as an add-on package."

I don't see what is particularly puzzling about implementing the things that matter to you personally, and leaving other things to contributions. I especially don't see a problem with leaving things to add-on packages if it's possible to leave them out of the core. Papers with complex math in them are a pretty small percentage of the overall number of books/articles in the world.

Also I like the aesthetic. YMMV.