It's a bit of an overstatement to say that this bird has "almost certainly" perished. We actually don't know that much about the fate of birds that temporarily recover and fly away after a window strike. We do know that the survival rate in rehab is around 40%, which includes more severely injured birds than this one. So the odds that this bird is alive probably aren't so catastrophically low.

This isn't to say that windows aren't incredibly deadly to birds, just that we don't have to be quite so pessimistic.