Agree: for most people, the beer gut is more pronounced because of the laxity of the front abs (also causing a hyper extended spine which causes back pain), it’s not all just fat. So, proper training and strengthening of the abs will help pull those things back in line, but won’t do much for burning off fat.
So, the rectum (poops last stop before freedom) is actually deep inside your pelvis as your colon makes the turn from the left side to the center, so your massage isn’t moving the poop out like squeezing a toothpaste tube. However, it lies close to some internal muscles like the iliacus which can put pressure on it if they’re stiff or inflamed (not unheard of in western society), and massaging them can get them to relax and relieve the pressure.
You’re certainly not hurting anything with this, and since you’ve been doing it for so long, you may have developed a psychosomatic connection that might actually help your bowels do their work. It’s weird, but go right ahead massaging the shit out of yourself because it might help in some way.