In the various Applied Engineering catalogs from the time period that I'm finding online, they were always proudly stating that they only designed products for Apple. So this does seem like an interesting mystery.
Not that one specifically, but others with the same bottom side labeling are unremarkable when viewed from the top. Standard NTSC 65XE case (no ECI).
I believe 65XE units made in Mexico by Sistemas Inteligentes are from 1985-1987.
Other "Made in Mexico" units have different underside labeling, which I believe indicates they were made by Atari (México) Fabricante, S.A. de C.V., which we know was established in 1987 (Atari SEC filings).
Later still, 65XE units for Mexico appear to have been imported from Atari "U.S." Corp. by Atari de México, S.A. de C.V. (stickers on some 65XE boxes), so I believe those indeed would have been from Taiwan.
I think that Grupo SITSA and Sistemas Inteligentes were sister companies operating at the same location, which were both acquired by Atari in 1987, creating Atari de México, S.A. de C.V. and Atari (México) Fabricante, S.A. de C.V., which both continued to operate at that same location.
Evidence is sparse, but this scenario would account for everything I've seen so far.