It should build with 1.0 since a recent update, but your other points are valid. That said, it's a PoC and kinda like a MVP. Granted this is from a domain expert, so the time it took Ralph Levien to code this is substantially shorter than it would have taken a beginner in the field of font rendering. However, it's easy to see that something like font-rs can be incorporated into Firefox in the future even before Servo is complete. It's already happening with image decoders.

To answer the parent's questions, which I unfortunately forgot to: if we translate Harfbuzz and FreeType to Rust, it won't take that long. See Dropbox's recent blogpost about their translation of Brotli's C code to Rust.