That article about Big Hero 7 and Tadashi being back is just a fan theory.
Producer Roy Conli said this regarding a potential sequel, which is not presently in the works and the fan theory article even says that they know a sequel isn’t in the works right now but just likely:
"It’s funny, I’m asked more questions about sequels than we ever actually talk about it. Literally, we finished this film on October 13, and we have been around the world three times, releasing in different markets, and the guys are – we have our big Oscar celebration, and we’re keeping our fingers crossed, and legs, and arms, and anything we can cross, we’re crossing – and we’re going to take a little vacation, and, probably sometime in June, we’ll reconvene and talk about what’s next.
It’s a big commitment to build a sequel. As creative artists, that means directors will be working with these characters for, at this point, probably nine years straight. So it’s a big commitment for them, so they have to think long and hard, and I think they need to make sure they have a story worth telling. If they come up with something that’s really good, it will probably happen, but the wonderful thing about working here is they don’t push us into sequels. It really comes out from, if the filmmaker has an idea, and it’s a good idea, that’s when it happens.”