Is a real rule but a nonsense call.
Think about other things deemed "unsportsmanlike," it's a high standard to meet and punishes stuff that sets the game back. Point of the rule is to keep someone from, say, faking an injury and then popping back up right before the snap, or starting a set with 13 players and then either pulling 2 WRs or a FB and a TE depending on how the defense lines up, etc.
It's been famously called on Quarterbacks that have trotted "confused" toward the sideline after the play is set, only to reveal they're actually a receiver and the RB to passes it to them. It's not fair to expect a defense to cover a QB who is clearly getting pulled or chatting with his coach about a playcall. But Pena was a receiver that moved to a different location, still a receiver. That's...normal.
It happened right in front of me, and we were all pretty confused and thought maybe the flag was for offsides. I'll guess the ref either thought Trebor was a QB, or was making up their own version of the rule. I'd really like to see a reply though, I can't seem to find the game on the ESPN app.