And they are Starfleet ships as they were wearing the uniform.
There are at least two other United Earth Starfleet ship classes in ENT's time: the 'Delta' type and the 'Intrepid' type. We don't know how old they are, or if there are any other classes besides these.
There seems to be some disagreement, for example, on where the Daedalus
class fits into all this. We don't know when it was first introduced or if it exists yet in ENT's time.