This was intentionally something the minions would do to try and stop Nanase from getting away, which is all the minions assume Nanase is trying to do.

It is not a case of Ellen the Game Master blessing them with knowledge they should not have to create a greater obstacle for Nanase.

Granted, given what Nanase has planned, it actually IS something the minions should be able to guess, but they're pumped with adrenaline and a desperate desire to not fail which is impeding their judgement also it's a game SO THERE HA