How much of that is auto-generated and pre-existing C code though? My dusty recollection from a while ago is that most of Tectonic's development is in Rust. It looks like the fork is trying to get rid of all the remaining C code?
That fork is quite curious, it seems like they've really gone out and created quite an active little fork, but haven't spent the time to explain what is different and interesting about their fork in the readme.