Is it possible to slide out from an HT nub design?

The tube fits a little tighter than the head of my penis, though it’s tricky to be sure since they don’t clarify which measurement defines the size. But after watching a short video (thanks Thumper), it appears you can pull the top of the glans past the rear edge of the device. Wouldn’t that make it easy to slip the tip out? Perhaps not all the way back in, but if I can get the ridge of my glans out, the rest should follow easily. Is there just insufficient space for the penis to retreat? What am I overlooking? Thanks.