> Machine learning excels at classification, and trash sorting is at heart a classification problem.

Classifying by physical properties is even easier by simply classifying by physical properties. Want to pull all ferrous metals out of a pile of stuff? Just use a magnet. Want to sort tin from plastic? Blower and conveyor bins by flight distance.

Incidentally, those elements not easily covered by this kind of sorting (which has been used for many years) is also not easy to cover by magical do-all machine learning, which will have a hard time sorting plastic-coated paper from paper from plastics, just like the processes above.