The difference is scale.

Airbnb isn't trying to revolutionize home ownership. They can have a successful business even if 99% of homeowners have no interest in renting out their place to strangers.

But when Musk talks about robotaxi, he doesn't describe it as a feature that some small percentage of Tesla owner will take advantage of and that Tesla will use to pull in some extra revenue. Instead, it's used as a justification to treat Tesla as a tech company rather than a car company, since robotaxi is going to completely change how society looks at transportation.

Even if Tesla successfully delivers robotaxi (a hard task in itself), there's no reason to assume it's going to cause the seismic shift that Musk seems to assume it will.