Dilution is a function of valuation and amount raised. Assuming they'd raise the same amount from public or private markets, valuation is the only factor in question.

It is not a given that public markets would price Airbnb higher than private markets. In fact, recent history suggests that the opposite often happens.

So no, IPOing is not necessarily less dilutive. Also, since their valuation is $20B+, dilution differences are a rounding error.