Wikipedia cites only some English language guidebook to El Tajin and a Spanish work, but who knows if those citations are actually related to the claim (wikipedia sourcing being notoriously dubious).

There is, however, a reference to "Xalapanecas" in Camargo's Historia de la Tlaxcala ( and appears to have been largely copied over into Torquemada's Monarquia Indiana) which talks about how the Aztecs conquered parts of the Gulf Coast including the lands of the "xalapanecas."

Xalapa is a real place, so this could just be a misspelling. But saying that the "xalapanecas" built El Tajin instead of the Totonacs doesn't make much sense, as the term just means someone from Xalapa. During the time that both Camargo and Torquemada are talking about -- the Late Postclassic, centuries after El Tajin -- Xalapa was settled by Totonacs, so the comparison doesn't make any sense.