Alternatively, it could mean there was a residual reservoir of cichlid fishes, of which the researches are currrently unaware.
Especialy so, since that would help explain where the fish came from in the first place, if the lake actually "completely" dried up. 300 species don't come from no ancestral population, but could easily come for a remnant pool of 300 ancestral species.
In Texas a species of snail darter (minnow) located in the San Marcos went on the endangered species list and caused about 5 Billion dollars in changes to the Texas water plans.
Strangely enough, these springs went completely dry during the lenghty drought of the 50s--and no one has successfully explained how these snail darters survived to become a listed species in the 80s.