For the latter part, remember M'Benga jury rigged its Transponder so that it wasn't sending a 'standard' signal, and I'll bet that's the method used by ship sensor systems to immediately identify another Federation vessel.So this ship is a legitimate Starfleet class, and not a kitbash? Then I wonder why they said it ‘seems to be’ a Crossfield, and why the Enterprise picked it up as an ‘unidentified vessel.’
