Not having sex doesn't make incels believe what they believe, believing/acting like that makes them not have sex. A lot of them don't just feel they're owed sex, they feel they're owed devotion. The fact that the prostitute doesn't actually like them and will only spend time with them the amount they're being paid will become a problem in their eyes.

And beyond that, the reason the hug helped this man is that it was genuine human connection. It was an act of free sharing in a way that Graeber would describe as an interaction of communism rather than exchange. Giving from one who has to one who needs simply because it's needed and easy to give. A prostitute is sex as an exchange, which includes both parties able to walk away as though nothing happened (indeed, the old saying goes that you pay the prostitute to walk away just as much as for the sex). Hell, many incels scoff at therapy because of its nature as an exchange. What they seem to lack is genuine constructive community, and often also the ability to actually partake in one appropriately.

Also I feel like it might result in a prostitute doing a mass shooting, but I can't defend that suspicion