I'm the second contributor of emacs-tree-sitter, a fine dude linked the mail thread where emacs-tree-sitter was mentioned and I went through it.
First off, in the issue tracker we have a ticket open for making this project available on GNU ELPA, Ubolonton seems willing to make that a reality but this package has some special requirements that would make impossible to submit it to GNU ELPA or MELPA, for instance, we need to ship with grammars for some languages that require compiling and AFAIK there is no way to build external dependencies in *ELPA.
That said, most GNU/Linux distributions ship Emacs with the modules feature turned off, AFAIK, which for me personally (and maybe Ubolonton would agree with me) is worrisome: I would like to everybody enjoy the benefits of this project without having to re-compile Emacs, not sure if this happens because Emacs itself ships with the flag turned off by default, clarification about this is welcome.
I have gone through th0rex's pull request and nothing suggests that hooks are being used in order to do the fontification of buffers, the only hook we have is `tree-sitter--after-change', it does re-parsing and keeps the tree updated; the plan seems to use font-lock. There are some "edge cases" with fontification like, for instance, multiple-language files, that the project still has to find an answer.
Indeed, projects like this and lsp have their entire development cycle confined to the comfort of Github, but (and I only speak for myself) it never crossed my mind to send an email here requesting any kind of help or advice, the time I needed a code review for one of my branch while the project owner was absent I actually made a post on Reddit. Don't get me wrong, I (and I presume this is the case for Ubolonton too) have no beef against emacs-devel or any of the maintainers, is that emacs-devel wasn't front and center in my mind when I had to seek help.
th0rex informed us that he won't be able to keep working on his pull request for some time, I think in some months are left to go, but after that he would present something better than what is currently shown. In any case, he had some issues with fontification and I would appreciate if someone here with more experience in Emacs Lisp and font-lock could take a look and comment the pull request. I understand font-lock is documented but for me at least was too dense to wrap my mind around it, I would suggest code examples would come handy, not sure if the suggestion sounds absurd.
Finally, I'm planning to tackle indentation for my next pull request. It come as a surprise that Emacs has no "central" indentation engine but that instead each major mode ships with its own indentation engine. Maybe someone here could point me in the right direction? it seems this entry in the Emacs Lisp reference manual about indentation covers the facilities common to each engine of all major modes.
Hopefully can eventually join the conversation. I'm looking forward to the replies.