In theory, yes: NT has always had the capability to have multiple subsystems. But apart from some attempts in the 90's that were aborted, it has gone totally unused up until WSL arrived.
A creator of Interix said that after he left Microsoft in 2004, he had to explain to the Windows HPC team that Interix existed. So totally unused may be an exaggeration, but it sounds like it was overlooked.
Not well known? Sure. GP didn't know about it, for example.
But "totally unused" is not an exaggeration, it is simply incorrect.
Thanks for the link. Interesting to read Interix founder and open source advocate's personal tales of interacting with early-2000s Microsoft after an acquisition that he didn't feel sufficiently compensated for. ;)
There have been Microsoft POSIX, Interix, and OS/2 subsystems.
https://en.wikipedia.org/wiki/Architecture_of_Windows_NT#Use...
EDIT: Re. "totally unused," Interix was available through Windows Server 2012, and supported through 2014.