Yes, I was about to say this in my comment! Due to a bug causing a 1 frame lag allowing you to open the menu because a trainer spots you and challenges you, but only when the trainer is looking down.
I didn’t pay attention to that kind of stuff at the time, but after playing Tales of Symphonia for years and finding numerous glitches I was surprised I had never noticed that in Pokemon. That said, I’m not sure if you could exploit this to actually walk past the trainer, or if you just have time to open the menu, which strongly reduces the interest of the exploit, until you understand that flying/digging out to interrupt the spotting sequence causes some bad memory state in the game (which would probably sound crazy if I hadn’t seen other glitches and learned game programming myself - although this is much less likely nowadays with object-oriented programming and such, as opposed to raw memory reading).
Would there have been such a rumour, we would have believed it and tried it… and quickly confirmed it was working. Thus it wouldn’t be a rumour anymore. Maybe the fact that a rumour, esp. of an easily reproducible action, stays a rumour after a few months is a sign that it’s not true…