Sorry if this is old news. It’s difficult for me to keep up with all the new info. Hopefully this is helpful, here goes:
When you do a special or super move with LP+LK, you’ll get a regular throw. For example, if you try to do a fireball with Ryu by inputting qcf, then pressing LP+LK, he’ll do a throw. The game will not recognize your special move, and instead just give you a regular throw. But if you input down, down/forward, forward, up/forward, then press LP+LK, Ryu will do a fireball; he will no longer do a throw. Furthermore, if you grab Ryu while he’s doing a fireball, he will tech your throw. This same mechanic applies to supers.
I don’t know exactly why this works, but my guess is that it’s because of a similar effect in cvs2. In cvs2, you can cancel your pre-jump animation frames into a special move, and as a result, you inherit throw invincibility. Here in SFV, I think by canceling your pre-jump frames, the game is allowing you to apply the LP+LK towards the special/super move, and it somehow disregards it as a throw–but it still inherits the ability to tech throw.