For the UNIX authors, that was already possible a decade before.

> however I will note that a large reason that C exists in the first place was to have a machine independent language to write programs in.

That's fair, but what we call a monstrosity by modern standards is much simpler than porting the assembly

There were cross plaform languages before C, but they never really took off for system development the wat C did (OSs, for example were commonly written in pure assembly)

A side effect of C not having a price tag associated with it, anyone with UNIX source tapes got a C compiler for free, until commercial UNIX became a thing, and splitted into user/developer SKUs, and thus GCC largely ignored until then became a thing worth supporting.