Single speed hubs are often speced with sealed bearings. In my brief market search I haven't seen any hubs with cartridge bearings in the specifications.
Are sealed bearings used synonymous with cartridge bearings in the context of single speed hubs? How to identify if a hub has cartridge bearings?
In general, the presence of a seal does not depend on the type of bearing. A cup and cone bearing may be sealed while a cartridge bearing may lack a seal. However, track bikes (velodrome) usually have cup and cone bearings for adjustability and lack seals to reduce friction losses. This may have led to such hubs' bearings being referred to as unsealed.
See also a related question on hub bearings.