> We believe that this is the largest rollout globally of any library written in Rust.

I think that crown currently goes to https://github.com/googlefonts/fontations which is included in Chromium, not sure if it's on all platforms yet. Moreover, the translative dependencies of Fontations (click through https://crates.io/crates/fontations/0.3.0/dependencies) should have an even (slightly) larger install-base.

EDIT: from the quote you can also gather that they don't use https://github.com/signalapp/libsignal