It specifically says it can kept indoors. I live in a place where it gets below -1. And they like the heat. Which I have it warm.
It might say all that. Yet it is not true.
One degree frost is absolutely no problem for lavender. The ones in the garden freeze up every year. Protected from wind and sun, they should be fine. Problem is, you now have it indoors so it has absolutely no frost protection. I would recommend putting it in a bright and cold room, as cold yet frost free as you can find.