I would fix that gap asap. If the B learns that it's a weak spot, it will never forget, and then it may take even more extensive measures to B-proof it. Even if it couldn't fit through now, it could probably dig out in 2 minutes. It's not enough to be B-proof now, you've got to stay a step ahead. I even suggest putting a trench of lava rock around the inside of the fence. B's do not like to dig through that stuff.
Under our fence all the way around is cement there is no way he can dig his way out. When we were looking into houses to rent we took having a basenji into mind (the owner even put a new taller fence in for us):) On the one side yard there is a grass area ut rocks all alond the fence over there (the ppl who live here before us had a dog we keep finding their doggy toys in the yard)