That's a good point - once the tank is full, the inlet is likely to be slightly larger than the vent hose, allowing more water in than can escape.
Just for fun, I put in a city fill bypass last night - about $8 in parts from Lowes.  It worked great, but did highlight some things:
Now the suction side of the pump is now under pressure, my inlet strainer & Winterizing plug needed to be tightened so they didn't leak. 
Yes, the tank did balloon a bit when it got full and started spraying out the vent, but I was watching to stop it before it got too far.  Opening the gravity fill would have been a good move, especially if I was using a higher flow water source.  Maybe I'll put a warning label near that valve to remind myself.
Normally if I walk away from a tank filling operation, I'll set my watch alarm for 5 mins so I remember to check it!
CC