Interesting article. I also can’t imagine that it’s either sisal or Americana exactly, both are so tough & fibrous, and are in such widespread cultivation that this preparation would be more well-known. The agave heart or “piña” used for mezcal is somewhat similar but not that similar. Agaves very quickly grow some massive asparagus-shaped flower stalks before they die, is it possible this is the base of an enormous stalk rather than a stem? Never seen one this big but maybe it’s out there.

I also would not discount the possibility that these are not raw exactly but prepared/treated somehow. Obviously the fact that they taste like water limits the possibilities, but maybe a stem is placed in water for awhile to make it way more engorged than it would ever become naturally. Or maybe it’s soaked or boiled after harvesting, and flushed with clean water to improve the texture and reduce toxic effects. Raw agave contains calcium raphides, a painful irritant (experienced by another commenter upthread, btw) so some kind of preparation might be necessary.

I always love a good nerdy mystery story. Good luck on your search :)