I think you asked the equivalent of “If a tree falls in the forest, and no one is around to hear it, does it make a sound?”

There are vibrations propagating through the medium of the tuning fork. Thus, sound is being propagated. In space it doesn’t have a medium to transfer that wave to, and thus it cannot be perceived by the human ear. That doesn’t mean the vibrations aren’t functionally the same as if we hit the tuning fork in a place with some sort of atmosphere. Sound doesn’t need to be perceived to be functionally the same thing as sound that is perceived. Technically any vibration through a medium is considered sound, whether you can hear it or not.

Wouldn’t the medium of the tuning fork propagate the sound within itself? Sound doesn’t propagate only through gas.