Well, if they didn't build their own computers, they certainly went to a lot of trouble to fake up photos of the family members doing electronic prototypes. And I think some of those photos may be older than PhotoShop, so faking them would have been more work than just building a home computer, though validating the photos’ apparent age might require digging up 1980s Mexican computer hobbyist magazines. (There's a convenient list of computer-magazine articles stretching back to 1981 on the Biyubi website; if you're in Mexico, maybe you could try to find them in a library or a used bookstore.)

Also they offer a course in building your own electronics that you can sign up for if you go to their workshop. I can't go to Mexico so I haven't signed up for it; it's at least theoretically possible that the course doesn't really exist and will be canceled if you do sign up for it. But if that doesn't happen you can go there and take the course and see the G11V3 and presumably Fénix and Biyubi in person. I've seen comments in news-website comment threads from people who claim to have done so and claim that it's real, and no comments from people who claim to have done so and then been rejected, so it seems most likely that the course is for real.

Together with Óscar Toledo G.’s amply demonstrated skills at programming under extreme resource constraints, skills which require a great deal of practice to acquire, I think the evidence strongly favors the hypothesis that Fénix, Biyubi, the G11V3, the electronics courses, and the rest of it is real, if maybe a bit oversold.