Wasn't Burroughs B5000 the first computer to have all system software written in a high level language in the 1960s?
[1]: https://en.wikipedia.org/wiki/Burroughs_large_systems#B5000
[1]: https://en.wikipedia.org/wiki/Burroughs_large_systems#B5000
(To head off complaints about what "portable means": portable enough that people did port Unix and C programs to many architectures, unlike the case with the Burroughs OS and programs.)
So the Unix claims of firsts need qualifiers to be absolutely true, but are true in spirit even when phrased casually: the earlier exceptions are mostly just curiosities.