If you're hanging your hat on that, there's not much else to say. I don't think I'll ever agree to that.
But I'll give you three things to think about and then I'm out.
1. I prefer Merriam Webster. More accurate, not as snotty.
https://www.merriam-webster.com/dictionary/propel
2. Even if so, "forward" is a direction that changes as our body angle and direction changes. A scant minority of BH throws are release while facing the direction you may call "forward". It can be argued that, at the time of release, that WAS forward.
3. As the Supreme Court said when prompted to "define" pornography, "we may not be able to put it into a nice clean definition, but we know it when I see it." In other words, old-fashioned common sense. Seeing the car in reverse, that's propulsion. And seeing Lisa's throw, that' propulsion.
Btw, what is your opinion of Brady's career altering moment? Objectively, I mean, please don't invoke NFL.