(I don't work for Airbnb, but work in a similar space)

Yes, this variable is usually set after the fact. For instance, a given transaction may have led to a chargeback, or may be done by a known fraudster. These models are usually trained on historical data, so we can know with some certainty which transactions are fraud.

It could be that a transaction was fraudulent but has not yet led to a chargeback (maybe the real cardholder hasn't yet seen their statement?), so there's still some uncertainty, but hopefully that approaches a minimum after some time passes.