It’s obviously a spectrum, but (some) people think of different things when they hear “closed source” vs. “source available”. Also, a company like Microsoft providing source to big customers doesn’t make their products source available, at least not in the commonly understood way. Source available usually means the source is freely available for reading and often compiling. That obviously does not apply to Windows.
Which, of course, is obviously false. Which means the words have different meaning than what you ascribe to them.
Source available means you can view the source code, by abiding to the terms the owner of the code sets.
Closed source means you can’t change or redistribute that source, regardless of whether you are allowed to view it.