even if the argument was that Bumphis never made a legit fair catch signal, he did make an "invalid signal" (I'm going off of memory here, the video is to grainy on my screen to watch his arms - if he made no motions whatsoever, this does not apply).
The rules are written in a way where invalid signals are treated basically the same as a fair catch signal - even when the ball hits the ground and a receiver signals everyone to "clear out". Basically, if you want to return the football, don't wave your arms in any unnatural fashion. There is a scenario exactly the same as this one cited in the rule book that clearly says you can not advance the ball. I'm too lazy to look it up again - somebody else can have at it...