Supposedly T. ellioti is the correct name, due to one of the rare exceptions in nomenclature. Usually it is the first, older name that is the valid one (in this cause maculipennis) but there are a rare few exceptions. In this one, the genus was described based on T. ellioti. It didn't matter when Thorichthys was condensed back into Cichlasoma, so it was still maculipennis. But when Thorichthys was resurrected as a valid genus, it gave ellioti precedence. Or so say the super scientific sites that are way above my league.
You may be correct about that, although the details of your explanation (and the finer points of nomenclature rules) are not clear to me. I was going by the discussion forwarded in 1996 by Juan Miguel Artigas Azas in an article available at Cichlid Room Companion. He argues the proper name is T. maculipennis rather than ellioti.