I think it's related to people remembering the dark old darks of a watch being labelled "water resistant" (meaning nothing really) and "waterproof 10m" etc.
Nowadays if a watch is rated to 3/5/10/20ATM and you trust the company doing the rating (and have serviced your watch/checked water resistance as needed) then you can expect it to perform to the rated depth (and probably 20% more).
In fact most 3ATM watches can probably be brought diving - the deepest I've dove was -28m but I didn't say so just in case someone wants to dive to -40m...
Here's Omega's own WR chart from a few years back - you see they group 3/5/10atm into same swimming category of water resistance:
I imagine the reason Omega don't advise you to dive with their 3/5/10atm watches is that they do not have the useful diving features of locking crown, crown guard, uni-directional bezel - nothing to do with water resistance.