I checked again and my copy of Chrome did not have built-in PiP on Win 10 Pro.
But the Chrome web store has the following PiP extension provided by Google so I added that to my Chrome install, tested it on a YT video and the pop-out video window could be snapped to my FancyZones grid.
https://chromewebstore.google.com/detail/picture-in-picture-extens/hkgfoiooedgoejojocmhlaklaeopbecg
I then installed that same plugin into my Opera GX browser and the pop-out video window could NOT be snapped to my FancyZones grid.
Thus, the underlying way that the Opera browsers are facilitating the popout-video windows, either natively or via other party's extensions, is now incompatible with FancyZones.
Just a bit of additional info for anyone looking on from a technical perspective.