FWIW, I use a trident-9 suppressor on a Glock 19. I shoot outdoors and only use subsonic ammo. The noise difference is substantial. My neighbor 100 yards away can't tell I am shooting if he is inside his house. Outside, he says it sounds like a nailgun. It's also hearing safe for me without earplugs.
I would suspect that indoors where he might have tried it, it might have sounded a lot louder than outside where the noise can dissipate without bouncing around. Also, subsonic vs. supersonic ammo makes a huge difference. Supersonic ammo sounds like a .22lr out of a suppressor. Subsonic sounds like a loud BB-Gun or nail-gun. If you reload, you can also tailor the ammo to the suppressor; some powders (I like N330) are significantly quieter, and cleaner in the can than others. I prefer 124gr bullets as keeping them subsonic generates less pressure, and thus less sound, but with the right powder will still cycle reliably.
That said, a suppressor isn't going to sound like the movies, but with a good suppressor, that Glock 17 is one of the best candidates to get as quiet as a handgun can.