The twitter thread doesn't go into any actual detail, so it's completely impossible to know what the situation actually was. But no unpaid internships last for two years. So the most likely scenario seems to be that they did an unpaid internship for Chucklefish (normal, still acceptable in the US sadly), but then somehow were able to continue working on the game, for free, after this internship ended. So there are big questions for Chucklefish on how exactly this was allowed to happen, assuming it actually did happen in that way.
While there is nothing unusual about an unpaid internship (unfortunately), it's incredible that someone could continue to contribute to a game without getting paid, after this period of glorified work experience had come to an end. There are serious safeguarding issues there, especially as the person involved was under 18. Who in Chucklefish was co-ordinating and delegating the work? Is there no legal regulation of unpaid work in the US? Someone at Chucklefish must have been saying "we need this work done- ok, we'll give it to this person who once interned for us, without paying". Even if that person is emailing every day saying "I want some experience! I'll do anything you want for free!", there needs to be ethical codes and legal regulation that absolutely prevents this from happening, rather than it being okay at the discretion of the developer.
If America's rigid class system means that unpaid internships are unlikely to disappear any time soon, they can at least be very heavily regulated.
This also highlights the educational role that trade unions need to play, and the importance of such education in high schools etc. You are always vulnerable as an employee, even if you're doing an internship and you're not expecting to get paid.
