Note though that this Tectonic is based on a fork of the tangled XeTeX sources (i.e. not the actual WEB source code but the machine-generated C translation) as

Note though that this Tectonic is based on a fork of the tangled XeTeX sources (i.e. not the actual WEB source code but the machine-generated C translation) as of some past date. This is a very weird choice probably due to the author (understandably) not being able to make sense of the mess of TeX sources to figure out what's the actual source code. As a consequence, already it has begun to diverge; for instance it does not contain the \expanded primitive that was added to XeTeX last year. Overall, Tectonic seems to have many good ideas, but suffers from being not too familiar with TeX itself and the TeX ecosystem.