Your example isn't comparable at all. The character was saying something that happened with them off screen, we are to accept under normal circumstances, of course. Here, we don't have information of Vegeta's awareness occurring off screen, only that he knew 17 witnessed a Trunks. The information of 17 being aware inside of Cell isn't hinted to have happened off screen, because there was nothing pointing at that. You assume it must have happened off screen. Your example would be comparable if Vegeta had said he was informed by Goku that 17 can see things while inside Cell. Therein, an off screen information (Vegeta's awareness) would've been made clear to the readers and there wouldn't need to be any discussion in the first place. However, it didn't happen, there was no hint of any statement from Vegeta or anyone that the prince was fed such information off screen, which you keep overlooking and that's why your analogies keep being incomplete, your analogies don't weigh at all to what you're proposing here... It remains simply as an speculation. For more that you'd like this to be a concrete power placement statement, which would be cool, you passing it off as a 100% certain thing is only leading to circular reasoning, heavy assumptions and weird rationalizations ("the Cell Juniors were holding back... But they weren't holding back by much... I know the extent they were holding back because... Reasons"). Sorry P, but you see many concrete power statements where there's none.