The presence of the barrier strongly implies Mizuchi still has access to Yukari's abilities somehow - or possibly it will be revealed Reimu is the one who put up the barrier around the temple grounds, but didn't tell anyone about it, including her own allies, to minimize the risk of Mizuchi finding out some way. It would make, considering how slippery she's proven to be.
It's very possible that because Reimu hadn't met any of the Myouren Crew before the events of UFO, she always assumed they all broke out at the same time. Or she knew once that some of them hadn't, but has forgotten by now. Or she figures that since they're all so tight with each other they'd have told each other everything they knew about vengeful spirits the moment Mizuchi became the talk of Gensokyo.