As I understand/recall - the intention was always to be a rust rewrite; the starting point was 'let's take all the C and wrap it in `unsafe`'.
The issue linked up-thread introduces this fork (~ a year ago) titling it '[convert to rust] everything' (emphasis mine), and going on to talk about 'pure rust' (again).
I think the main repo's issue at least would've been a clearer submission than the fork; I initially had the same reaction as above.
https://github.com/tectonic-typesetting/tectonic/blob/master...