It looks like this is a reference to Eugène Beauvois, a nineteenth century French writer who connected Mexico to the Celts by identifying Plutarch’s references to the island of Kronos with Mexico. He apparently made the claim in the 1902 journal article, “Les Templiers de l’ancien Mexique et leur origine européenne.” I have not had the time to read this article in full, which I only just learned of this morning, but the claim appears on the very first page, so I guess that makes it the origin point for all the later versions: “The last of the emigrants who introduced into ancient Mexico the notions of Christianity and of White men carried the significant name Tecpantlacs (Templars)…” (my trans.; parentheses in original).
The specific text cited in support is that of Chimalpahin, whom Falardeau refers to by some of his Spanish names, San Antón Muñón. Chimalpahin was a Mexican writer of the sixteenth century who wrote a history of his people in Nahuatl. His Annals were translated into French and published in the nineteenth century. Falardeau and Beauvais, along with other writers such as Emilio Corbière and (in America) Steven Sora allege that Chimalpahin (or, more rarely, an unspecified “Vatican codex”) record the arrival of the “Tecpantlaques” in the middle 1200s and they etymologize this word as coming from tecpan (“temple” or “palace”), suggesting that they were the “People of the Temple,” and thus the Knights Templar.
I’ll be damned if I can find any such word as “Tecpantlaques” in Chimalpahin’s Annals. The relevant passages that the authors seem to describe refer to the “Tlacochcalcas” who, in the entries for 1272 to 1274, crossed the Great Sea after leaving Aztlan en route to Mexico, coming through a snowy land. Such references our authors want us to see as references to Europe and the Atlantic, though the plain meaning of the text is that the arriving people were future Mexican nations coming from the north.
The closest I can find is the two-word phrase tecpan tlaca, which is translated as gens de palais (“the people of the palace”). According to Spanish-language sources, a tecpantlaque was a member of a special class of Aztec workers who maintained a lord’s palace or a god’s temple, but this is a particularly rare Europeanized spelling, and I think our version is actually a Francophone author’s attempt to turn tecpan tlaca (modern: tecpantlaca) into a French word by Gallicizing its terminal vowel. This is especially strange because, after researching this, I discovered (as you can see above) that Beauvois actually spelled it the right way, and his copyists somehow got it wrong.
So how did the tecpan tlaca get promoted to an invading force of Templars? That’s a fun one involving the transitive property. In the introductory remarks to the seventh book of his Annals, Chimalpahin writes, as I translate it from the French and modern Spanish editions:
It is said that the Nonohualcas, Téotlixcas and Tlacochcalcas were not Chichimeca, but they were called the “people of the palace,” because they gave themselves over to and were servants in the house of that devil [i.e. god] Tlatlauhqui Tezcatlipoca … (my trans.)
What is perhaps most interesting is that this same set of material from Chimalpahin had also been used (apparently independently) by T. S. Denison in 1908 in an effort to prove that the speakers of Nahuatl were actually “primitive Aryans.” “One thing is certain,” he wrote in The Primitive Aryans of America. “We must dismiss all notion that the Nahua developed an indigenous civilization on American soil in spite of assertions to that effect by prominent writers. They distinctly inherited the old Aryan culture of Western Asia.”
Apparently everyone wanted to find white people in Mexico!