Contrary to popular belief, a silencer does not quiet a gunshot sound to a tiny pooftt sound.
Correct. The average “silencer” quiets the dound down by about 30-35 dB.
Subsonic .22 ammo with a supressor is pretty danged quiet, but generally speaking, you are correct.
For normal factory loads, that is correct. But custom and very lethal loads can be hand loaded that are as quiet as a BB gun.
You can already buy “silent” 22lr ammo. It is actually quieter than a BB gun, no suppressor needed when fired from a rifle.