Houthi drones can and do fly 900 miles carrying significant ordinance, it's happened multiple times.

They started on their drone journey buying cool consumer drones on aliexpress for $100 or so and modifying. They have access to the internet and enthusiast blogs and have rapidly grown their industry. Obviously unlike quadcopters they don't rely on wifi but rather use other RF bands. These people have lived there in the desert for tens of thousands of years. They are smart people and able to understand and improve on designs.

It's not particularly sophisticated, liek on the Manhattan Project level requiring state backing. There are kids doing this stuff for fun without their parents even realizing.

They'd need a really tall tower to remotely pilot something 900 miles off with no satellite. I think autonomous flight via gps is more realistic.

The GPS and inertial gets the drone close enough. During the last stretch the Houthis have stated they have local guidance assist. The population in the area of the targets is significantly Shia, anti-KSA, and revolutionarily minded so it's not surprising this resource is available.

The article points out another successfully hit target was the Patriot search radar. The fiberglass airframe indicated in your link supports that capability. Autopilot code is as mentioned elsewhere open source. Scary stuff