Without commenting on your larger point, it's doubtful that Yemeni engineers are building these drones.

Yemen is one of the poorest countries in the world and presumably their engineering capital is presumably not huge. The Houthis are backed by Iran. Iran (which has far more engineering capital and was accidentally gifted a US military drone several years ago) has military drones.

It sounds far more plausible to me that Iranian support is how the Houthis obtained these drones.