Unless it was established ahead of time, I don't think the DM thumbing the dive is the same as a general recall. I'll agree that if you hear/see a general recall signal you'd better get your ass to the surface, but without more a DM's thumb doesn't carry that kind of weight.
The idea that a thumb is a recall has a few problems: first, it would be a pretty ineffective means of initiating one and there's a reason beating the Hell out of the ladder with a wrench is generally used. Second, if you hear a general recall you'd better get out ASAP, which to me means blow off any non-essential delays like safety stops.
If someone general recalls me just because they want to exercise their perceived authority to end the dive, I'll still be out of the water right away. But if I get back on the boat to find there's not an emergency that justified me blowing off whatever approach I'd usually take to exiting the water, they'll spend the rest of the trip back to the dock getting their ass chewed off followed by complaints to their shop owner, BBB, certifying agencies, and any insurer I can track down.
