You really think a QB getting all first team reps would be kept secret for a week, by all players, coaches, equipment staff, PR staff, the grounds crew, etc.? No chance.
It wouldn't, but I doubt the Bucs would game plane based on rumor. Where as if you announce it, they would.
That being said it's not a huge deal. They likely already have their game plan in. They could tweak it, but there isn't any film on Rattler in this offense anyway so it would be hard to really plan against him.
And as Bresusaves said above this will get the crowd into the game and keep the locker room energy good this week. They really needed a boost after three straight loses.