This is addressed in the comments on the linked page, the browsers on those platforms do support the required codecs and all that's required is a user agent change to get the xbox end to activate them.
This could also be explained by Microsoft not doing their browser sniffing well or realizing that the Linux browsers supported the other codecs. If we’re talking about proprietary codecs running on Linux, then the fact that they work at all with Xbox cloud gaming is a good thing. If the codecs have a quasi-legality to them (e.g. proprietary, but fork exists — no clue here — I don’t know the codecs), then this kind of unofficial support might be the best Microsoft can do. Otherwise, how can you detect a codec from a user agent?
My guess is that they are just lazily using User-agent as a proxy for codec detection. I don't think there is a reason to jump to a malicious intent here (even with existing priors).
