Hmm, yeah going to have to say that these might be just prop ships.
You're forgetting this is the start of a new series of canon. Whether we like it or not, if they choose to embrace it and run with it, we have no choice but to accept it as part of the post-halo 3 UNSC. That's different from the plot device Super Carrier as that was a (as of now) one time addition and seemingly a product of laziness. If more information about these new frigates becomes available, I'd be curious about adding them in.