The Windows button doesn't do anything to Inkscape, for me. (Just causes the Start menu to pop up on top of Inkscape, which is expected.)
I can't tell for sure, because the status bar is behind the video timer bar. But it looks like they are holding the Alt key, to me. Yes, the Alt key does provide the behavior shown in the video. Apparently that's a lasso select, which I'm pretty sure was purposely programmed, and not any kind of easter egg.
But I don't think the lasso select is well known. If someone had asked about it, I would have had to look it up.
On my keyboard, the left Alt key and Windows key are right beside each other. Maybe on some keyboards the 2 keys are combined into 1, for some reason??
Interesting though. When I press the Windows key with Inkscape open, the status bar shows the Alt key info. That's a bit odd! Maybe there's some mistaken crossover for some computers?