CEMEX makes and distributes Cement.
Their plants are mostly located at transportation nodes and/or near major urban centers, construction areas or other major capital investments that use a lot of cement.
So I doubt CEMEX has a plant on an almost uninhabited island.
CEMEX is reportedly owned in part by Soros ($41M in stock), with ties to Carlos Slim, and CF.
The island has some development. Developers always have cement stored in some form. Cement can be in dry form and housed in covered structures.
It’s not implausible. I would not weigh the rumor is false until I knew for sure CEMEX had no presence and nothing stored on that island. I could go check it out myself but I’m too lazy, actually too busy. FR is among my free-time activities.
Unless the military or oligarchy had use for a cement plant out of prying eyes for various black projects—nearby or some such—depending on how true the deep tunnel system is.