Labour aristocracy can refer to any group of privileged workers. In analyses of Imperialism, all white workers in the imperial core are considered the aristocracy. Other times it's the most experienced and skilled workers working to defend their status from others.

PMC is often defined by their education, but a more Marxist definition for the same group would be "people who are removed the means of production, but do not own them". As I see it, they primarily rely on their knowledge, skills, and ideas to organize capital and labour for greater efficiency, but are otherwise outside the chain of production. On a small scale, this is management. At a larger scale, it's politics, ideology, propaganda, advertising...

Even though they're not bourgeoisie, they're doing the work of the bourgeoisie. Reproducing and maintaining the systems of capitalism. They ensure profitability and success in the market place, but they do it for someone else.