Yes, Buddy, you are correct in that the catch can is there only to prevent excess fluid from leaking onto the road. The Harrison tank is for holding normal expansion of fluids. It is interesting how differently the expansion tanks can be plumbed. Mine is way different than yours.
Is your concern theoretical (this may happen), or have you had issues with the coolant in the Harrison tank overfilling it and dropping it on the road? In all the time that I have raced mine I have only had a small amount of water get deposited in the catch can.
Good luck.
Jim