The fundamental issue is testing, how do you acquire test environments that old and what exactly is gained by the continuous effort required to keep them running when their Apple themselves refuses to do so?
Basically what you said.
Not so much "this could never work" as much "we can't test it so why claim support". And it is a can't test not won't test. Seems the software delivery mechanism doesn't officially support pinning old versions in a way that Rust is comfortable using.
Put another way if an end user can't build an old version from a new machine supporting that old version for people who haven't updated at the cost of workarounds isn't a good plan.