That not true is not true.

Rust For Linux has a hard dependency on unfinished Rust features that were not released as stable in the Rust version that Debian has.

However, every build of rustc also needs to be able to build its own standard library and a future version of itself. To do this, rustc recognizes a special private env var that exposes compiler-internal language extensions and not-yet-stabilized features meant for the compiler itself, and not for end users.

Rust for Linux relies on using this private env var to bypass feature stability checks and trick the "stable" compiler from Debian into allowing use of compiler-private and experimental features that were suppressed to stay disabled.

RfL uses an old Rust binary from Debian, but still depends on experimental features that haven't officially shipped yet in any Rust version.