It wasn't technically lying if they just left out parts of the truth, right? "I... I just realized. Flowey's all alone in Hotland. If Undyne catches him..." they bluff. They were absolutely worried about Flowey, but that took a backseat to the blinding panic that came with the sudden removal of quite possibly the cushiest of safety nets in existence. There was no going back now. "And, and, without us, he won't be able to get the SOULs and free everyone! I don't think they'll just let some random flower take them just because he asked nicely."