Numerically you're right, but you're showing a distinction without a difference. In the kinds of neighborhoods where Airbnb has a foothold (gentrified, desirable, centrally located) rents are already so high that an extra $68 doesn't make a damn bit of difference. I live in one of these neighborhoods, and I speak for absolutely everyone who can afford to pay the rent I'm paying: we don't care about the additional $68.
Those for whom $68 really is 5% of net monthly income don't live in Airbnb-infested neighborhoods, and thus are even less affected by the impact Airbnb has on rental prices.
I don't mean to sound insensitive to the plight of those for whom NYC is too expensive. I think such people need to be helped, but spending even an iota of thought on Airbnb is a waste of time: its impact is concentrated in neighborhoods that are already too far out of the reach of rent-challenged New Yorkers. And this isn't the case because of Airbnb.