No idea. But at least this is empirical data from a lot of samples. What if we had the results of learning difficulty for native speakers of all the major lang

No idea. But at least this is empirical data from a lot of samples. What if we had the results of learning difficulty for native speakers of all the major languages, and some languages were hard for everyone? In principle, this is an experiment that's doable, if the foreign ministries of the other countries also publish this data.