"To be specific, you need a vertical velocity component of the racquet's swing path to generate topspin. Force alone, at least on the racquet, won't do it. You can put all the force you want on the racquet. If there hasn't been enough time for that force to sufficiently accelerate the racquet upward to create that significant vertical velocity component you're not going to get much topspin."
Physically speaking, the top edge moving forward is also the back edge moving upward.
You do not need, as a matter of FACT, the racket to move upward in any way to create top spin, even if it does.
Top spin is also forward spin; it's also downward spin; it's also backward spin... it depends on your point of reference on the ball. In truth, they're not different: they're all the same thing happening at once.
Now, if you hit the ball slightly behind, all the while being above center, both upward and forward action creates top spin. That's what it is, period. You can start trying to spare your convictions as much as you want... I never said that it doesn't create spin -- I said that additional upward movement affects top spin marginally, for various reasons.
As for your impression regarding videos, all pros present a very vertical action once the contact is already made.