I think you misunderstood the meaning of the word "SHOULD" in the spec.
It means it's not strictly necessary, as in, a PRNG is not a requirement in order to support UUIDs in a compliant way.
To me this means UUID itself is not a viable solution if randomness is a requirement for you, because even if one claims they are using a UUID implementation that is compliant with the spec, and it is in fact compliant, that doesn't mean it's actually random at all.
It means it's not strictly necessary, as in, a PRNG is not a requirement in order to support UUIDs in a compliant way.
To me this means UUID itself is not a viable solution if randomness is a requirement for you, because even if one claims they are using a UUID implementation that is compliant with the spec, and it is in fact compliant, that doesn't mean it's actually random at all.