To be clear, it's not so much they are using ML to predict wind speeds at different altitudes; they use a forecast model like everyone else for that. The clever bit is being able to control your altitude such that you find favorable wind-altitude tuples to be in to keep you generally in one spot over days/weeks. It's sort of like what ocean/circumnavigating sailors do but in a higher dimensional space, both literally and in your search algorithm. Fascinating stuff.