I seriously doubt Nintendo would entrust Grezzo with a follow-up to their GOTY. They really don't have the design pedigree to make a game like that. Ports using Nintendo's source code is one thing, a sequel to GOTY sounds unusual. Especially when the series is on track to selling 700k+ in Japan. Nintendo is going to want to push the sequel back into a million sales territory.
Nintendo was just hiring for a new 3D action adventure game. Most likely they are staffing up for BOTW team split into sequel and new IP? Maybe?
That seems hard considering that the core Zelda team is like 100 people.
Going to need a lot of hiring or a lot of outsourcing to Monolith Soft. Hopefully they could pull it off, but this could just be routine hiring to start a new game (whether that be a new IP or BotW 2 instead of both).