DSSS is magic. Truly. It's like you can create an ~infinite number of "new frequencies" to send on, each being orthogonal to any other, then chose them cryptographically as needed. With enough care it's possible to make it difficult for the adversary to even detect the presence of a signal. And the tech used by the likes of F-35 with MADL and low-probability-of-intercept radars may be even fancier than that.
That being said in the Red Sea incident they likely were not stealthy at all. For one F-18 has a rather big cross section, it's a 4th generation jet. For another the Houthis are not known to operate any sensible air defense (I may be wrong here, someone please correct me if I'm wrong).
Any serious hardware Ansar Allah operates is provided by Iran, and Iran is not capable of shooting down IAF aircraft flying over their own territory. Ansar Allah did not shoot down an F-18.
That being said in the Red Sea incident they likely were not stealthy at all. For one F-18 has a rather big cross section, it's a 4th generation jet. For another the Houthis are not known to operate any sensible air defense (I may be wrong here, someone please correct me if I'm wrong).