I think it’s more of a general confidence that there really is no system capable of detecting it. It flew halfway across the world, and radar systems usually have very high ranges. It wouldn’t necessarily need to be the Houthis that have the system.
If you remember, they didn’t let Turkey buy F-35s because they also had S-400s. They didn’t want it to be “public” knowledge how they match up. The US now knows, hence more confident stealth aircraft deployments.
It’s just my theory anyways.
Just as the US/Israel are brutalizing the Middle East. That’s the point.