Well it must not be true 100% true... But that's no different than saying the same for all texas cichlids or saying that all female red devils/midas/midevils have a rounded anal fin. It is more common to see these traits on females but it does not always mean female.
But you gotta understand the difference between the males black and females black. If that male's dorsal stood up, the black may not be a whole big solid blotch
The color can fade in and out in females too, depending on mood, dominance mode and status, or whether spawning or not.
Below is a female festae
and again
during spawning many males of the Heroine cichlid tribe (Which festae and carpintus belong)will show black in the dorsal
Below is a normal male carpintus
but when guarding a brood he and the female are very similar, the male on the right, is the same male in the pic above
Just wonder when will the black blotch develop in females. Do you have to wait till the females approach sexual maturity before the black blotch shows up. Obviously, if the fish is too small, you have no way of observing the dorsal fin clearly. So what's the minimum size fish when the black blotch shows up.