Considering that Picard seemed to hastily gather his fleet from wherever he could get ships (i.e. the Sutherland was still in dock being repaired, and didn't even have a commanding officer when it was called to duty), then lumping an old available freighter into the mix is plausible.
However, there's still the issue of the ship looking absolutely nothing like a typical Starfleet vessel, and looking more
like an alien ship, or at worst, a ship from an enemy of the Federation!
Of course it's also possible that the Hermes is not this type of ship, and there's a distinct difference between the Starfleet Antares class and the Federation Antares class.
(wondering if a new thread topic is needed about now...)