I think what you're missing is that this is likely in preparation for some sort of AR glasses and something smaller form factor than the Vision Pro that will require transparency so you can see through to the real world etc...
... could be the point of Liquid Glass on Vision Pro is to make AR on Meta's platform look bad. I mean, the MQ3 is a medium-spec smartphone on your face, the Vision Pro is a laptop on your face. The latter can support a much fancier UI without slowing down to the point where the video lags and you start crashing into things.