PDFs can be translated to EPUB using existing programs. I'm going to go out on a limb and guess that Typst emits "good" PDF (I worked on a PDF program for years and I know it when I see it) so that conversion should be fairly high fidelity.
Clearly this is not as good as offering a second backend, but the latter is also a huge amount of work. I'm sure if you showed Typst cases where their PDF output doesn't translate to EPUB cleanly using available converters, they would at least investigate and see if they can improve that.
But without demonstrating that translation isn't working, I'm not going to accept the claim that there's an accessibility problem here.
Feel free to step up and close it. Complaining about it isn't going to add velocity to a new project.