I just updated Boulder's boundaries from city open data (hand verification, not bulk import), and while there have been quite a lot of annexations since the boundaries were last updated, this isn't one - it remains a county enclave. Someone must really be holding out...

However, in this case, the "address" is still Boulder, I'm sure that is what is on all their envelopes. I wouldn't consider this the same as a case of a very remote house that might truly be in unincorporated county area with no obvious "city" for addressing purposes.