Salesmen are by and large:

1. Ignorant
2. Will lie to make a sale.

So the wire from the generator to the 20amp twistlock can handle 30amps. The wire from your 20amp outdoor box back to the breaker panel can handle 30amps. The only things rated 20amp are the twistlock and box. Since the prongs are indeed the same size (different orientation), you might believe that you can get away with it. However, inside that 20amp twistlock receptacle, there may be smaller pieces of copper between the female slots and the screws. Can they handle more than 20amps? I doubt it. That's why they make 30amp twistlocks.

Will it instantly melt if you push 21amps through it for 5 minutes? Probably not. 28amps for 12 hours? Might have a problem.

If you are backfeeding your generator into your breaker box (which I think that is what you are doing), that is very dangerous. It relies on you cutting the main to prevent backfeeding to the pole (and killing your friendly lineman trying to get power back to your house). Try as you might to always remember, you continue to see reports of home owners and linemen getting killed (or worse) with people backfeeding a generator via a plug and breaker.