Why not? From their point of view, this is a golden opportunity to get rid of her as PM. They can vote against her, get rid of her, then reform a government with someone who does hold the confidence of the house. A vote of no confidence does not automatically result in a general election.
You're correct in what you say.
But there is no one in the tory party who can take over and unite them so for that reason, they won't topple May just yet. You need a leader waiting to take over immediately to form another government to avoid an election. I don't see any way May can be toppled tomorrow and an election isn't called.