I saw that Reddit post a little while ago and my first thought was “There’s no way this happened. This is just some dude writing out his fantasy”. The whole thing is just not believable at all.
Anyway, yeah, men can orgasm just from anal stimulation, since not only is there a ton of nerve endings around the sphincter/anus entrance but men also have prostates, which is their own little pleasure spot. However, it usually takes more effort mentally and physically than just rubbing one out. I’d say it’s much more satisfying for a guy to have anal and genital stimulation at the same time. But, I’m not a guy so I can’t say for sure how it feels. I am, however, someone who enjoys being on the giving side of butt play with men, and judging from the reactions of partners who I’ve done this with (as well as from clip/cam site members who I’ve talked to, since I make a fair amount of content about doing guys in the butt) I’d say most would agree on stimulating both at the same time is the way to go rather than focusing on just the booty. Then again, some guys get off to the idea of a “hands free” anal orgasm. In the end, it totally just depends on the personal preferences of each individual person.