In that phase B has an intersection fully surrounded by live stones, so he could claim a point of territory.
Maybe you meant adding a special atari rule that takes away territory surrounded by alive but in-atari stones? That sounds like a dangerous ad-hoc thing, and I’m not sure how it would be better than excluding sekis (which gives a general solution to the whole problem, including other multi-state issues).
Btw, I see this rule simply as you need to have complete control to claim territory, which means pass-aliveable regions (LJRG by Pauli should also be mentioned).