Also, .22LR ammunition is generally subsonic outside of hotter loadings. This avoids the characteristic supersonic crack of higher velocity cartridges.
(as an aside, subsonic ammo is looked at suspiciously around these parts. since it's mostly used for poaching; when used with a home-made suppressor on a rifle, it's very quiet, like you say, in large part because of the lack of sonic boom).
Doing a bit of googling, the .22LR doesn't gain much velocity beyond 14" barrel length, and loses less than 200fps from a 4" barrel vs the 14".
Well, given that .22LR out of a rifle length barrel is less than 200FPS above subsonic speeds, that kind of plays right into what he was saying.
Now, you could use one of the other rimfire variations (.22 Short, .22 Long, .22 CB) which generally will fire from a .22 LR chamber. These would most likely get you subsonic velocities.
Another thing to consider when shooting a pistol vs a rifle and comparing sound signatures is that a pistol will eject more unburnt powder into the air around the muzzle as its had less barrel length to burn said powder.
As far as suppression goes, there are four main components of the sound signature of a gunshot:
1) muzzle blast - the expansion of deflagrating powder in the atmosphere as it exits the barrel.
2) sonic crack - the bullet creating a shockwave through the air. Can be avoided by using subsonic cartridges/loadings.
3) the weapons action - at the very least the firing pin's impact on the primer. This is louder than you'd expect.
4) the bullet's impact - very much louder than you'd expect. A rifle bullet impacting flesh is a very loud slap. Imagine dropping a large boneless beef roast on a tile floor from a few meters up.
Of these bits of the gunshot, only 1) can be addressed with a suppressor. And it does make shooting much more pleasant. You can generally avoid most ear protection shooting suppressed, even with supersonic ammo (you won't be in the shockwave so no need to worry about it).
I personally have fired rather large rifles without ear protection, and its generally not real pleasant. I really wish that there was no NFA and we could buy suppressors like some countries (New Zealand comes to mind). A .22 suppressor there is something like $40NZ, here you're looking at $4-600, a $200 tax stamp, and a 6month wait for the BATFE to do a background check that the FBI does in less than a week. Oh, and you can't let your wife/kid use it without being in violation of the NFA unless you have it owned by a trust where the family memebers are trustees.
The only stuff that's consistently making supersonic out of the list is the high velocity stuff. Even Remington Golden Bullet (which Remington advertises as "high velocity" according to the box I have sitting on my desk) falls short of supersonic from pistol length barrels.
I guess the fairer thing to say would be that .22LR is borderline but it drops below supersonic pretty quickly once you start shaving barrel length.