As the event post explains, most NPCs are being robbed of their will to fight by the Innocence Entity, so they will not be there to help ward them off, and the military strength of Achamoth is mostly irrelevant. The goal is, indeed, to lure Estinien towards Himeka and Abel so he can be captured, so it wouldn't be very effective to focus on keeping the Pleroma outside of the city entirely. It's a trap, essentially.

That said, it's a PVP event so it's assumed there will be pushback from both sides. It wouldn't be very interesting if the Pleroma just flat out couldn't get into the city period, though. There isn't anything about Achamoth that makes it completely impenetrable if most of the NPCs are out of the situation.