Ahhh, excellent question!
In the press conference Wednesday, they described their methodology as using exposures of such long duration that had their been fainter dwarfs, they would have been detected. In other words, the minimum detectable magnitude was below that of the faintest dwarf they saw, sufficiently lower such that they feel confident that they would have seen them if there were fainter ones.
The new Hubble camera is MUCH more sensitive, and they plan to use it to look for much fainter dwarfs (to verify that what they found WAS the faintest (and thus oldest), and to look at dwarfs in a completely differnt type of cluster, and in cluster much more distant.