Yeah, I don't believe that, either. Stringed instruments were commonplace, and the instrument makers pretty obviously knew how they worked.

Just guessing (I'd love to know the real reason):

Just because we know the object vibrates when making sound doesn't mean we know that sound travels through the air as a bunch of vibrations. Or that are ear is picking up vibrations. Maybe they thought the vibrations were something like the heat given off from friction. Not necessarily the main thing, just a side effect.

For all we knew maybe it could be some sort of "sound light" or magnetism like thing. Someone had to prove that it was vibrations all the way from the start to the end.

Loud sounds make nearby things vibrate, which is easily felt. Even louder sounds, like cannon fire, had obvious concussions and visible shock waves, and you could feel the thump in your chest.

A bell ringing will set up visible vibrations in a nearby cup of water.

Instrument makers shaped sound in air boxes in violins, and shaped it in horns. Sound has obvious propagation delays through air.

Maybe lay people had strange notions of sound, like flat earthers, but educated and observant people would know better. The evidence was all around, it did not need a lab experiment.

"The evidence was all around". -- well that can be said about anything. I'm sure 100 years from now there will be a similar post saying "the evidence was all around for faster than light travel" or the like

Also, human throats vibrate when speaking or humming. Drums, bells, triangles, and plucked string instruments have visible vibrations.

And the eardrum itself with a diaphragm is clearly set up to detect air vibrations, not some mystical force.