I don't think the comparison to Theranos is accurate. Nikola may be overvalued, but they do have (billions?) loads of pre-orders for hydrogen trucks. Manufacturing the vehicles isn't rocket science - lots of companies are manufacturing hydrogen buses that are in use in EU for example. The tech definitely exists, the same way battery tech existed before Tesla. Hydrogen makes more sense for trucks as it's lighter, and by outsourcing manufacturing most of the risk is removed.

PS Email me if you are interested in building a hydrogen powered flying vehicle.