It's not "open-source" under the usual definition. It's "shared-source".

To be fair, this is a term invented by Microsoft and only used by them, but I think it's accurate.

Well, the source is only "open" to subscribers: it's in a private Github repository. Stop paying, and you stop getting access to it, and it's illegal to publish even for perusal.

Exactly: the source is "open" to paying subscribers, just as the source to... just about any closed source software anywhere, can be seen if you buy a licence to see it. It fits NO definition of OS that I'm aware of.

Oh, I was mistaken. I thought it was available for anyone to browse, and you just couldn’t use it unless you bought a license.