I've never had any exposure to hardtack, but after reading about it, I'm thinking the big difference between bannock and hardtack is that bannock isn't intended to be made & then transported and/or stored.

Normally with bannock you'd bring the dry ingredients with you and then add water & cook it in the field (either in a fry pan or around a stick over the fire). When cooked, the bannock has a golden brown outer crust but is still soft inside, sort of like a biscuit.