"There are thousands of volunteers who donated their labour to Duo... Bit by bit all of our work was hidden from us as Duolingo became a publicly-traded company."

Oh look, theft that's legally protected because something starts making profit! It's like the existence of the stock market is the central problem with capitalism, since it's just an excuse to be a shitty person and is only ever used in that capacity. "fuck you, i get more money this way" is a dumb fucking principle to operate a society on. Antisocial, in fact. Google's IPO can be directly traced to every single problem the internet (and so, society) has right now.