What if this is simply more mind games by Golden. Just a hunch, coaches feel comfortable with Heaps and his progress thus far and wanted to test Kaaya out by not telling him/the team about Heaps sitting out until right before the scrimmage in order to apply as much mental pressure on Kaaya as possible. Coaches do this quite frequently at other positions during Fall camp to test backups out but it is unorthodox to do so with your QB who from all accounts is still learning the Offense. But given our injuries and suspension woes AG might have wanted to throw Kaaya into the fire now just in case we need to actually throw him into the fire come Labor Day.
I can see AG doing something like this IF he is comfortable with Heaps development thus far and he simply used a minor injury as a chance to see how Kaaya responds under pressure.