C is more portable than Rust when you define portable as "it compiles". If you define portable to mean "works correctly" I expect C is about as portable as Rust in the context of most applications.
I suspect you have a more narrow definition of “works correctly” than the C advocates do.
It wouldn't surprise me at all of most of those just crash-on-launch - or crash when you try to do anything, even though the CI builds them just fine and they pass the automated tests.
For an awful lot of boutique platforms, it's usually "literally one person" that drove the work to port it to that thing, and when that person's no longer actively doing the work, bitrot goes wild.