Yeah, that's it. But I wouldn't suggest disconnecting it just for having a look. Because, valved or not, it may not be a lone factor in the low mass air you see. It would only explain the symptoms noticed when removing the cap.
With that, if there is a lot of pressure/blow-by coming out of the crankcase due to age and wear...I'm thinking this could maybe reduce the mass air volume you are seeing. Increase in blow-by air will go into the turbo inlet and possibly reduce the air flowing over the MAF. Just a theory. Hmmm...this one is going to be a thinker. You still running an old MAF? I would only use a genuine VW MAF if it's replaced. But try the leak testing.