The author doesn't explain exactly what their interest in bootstrapping is, but the goal is pretty explicit:
So, for me, it would be really nice if there was a Rust compiler that could be bootstrapped from C. Specifically, a Rust compiler that can be bootstrapped from TinyCC, while assuming that there are no tools on the system yet that could be potentially useful.
Okay. That's just imposing a different (and at least equally arbitrary, if not moreso) definition of bootstrapping. Why does it matter that the author didn't explain their "deeper reasoning" for having an interest in bootstrapping, or the Bootstrappable Builds project specifically? If you feel like that project isn't meeting a sufficient bar for what counts as "bootstrapping", or that using C as the first high-level language they bootstrap is "tragic", take it up with that project, I guess.