Hundreds of people worked on that game, as many as some AAA games, and yet games like Blue Prince, from a solo developer (or very close to it?) had to compete against that?

Moby Games lists 121 people in the credits for Blue Prince and 416 for Clair Obscur. At some point, the number of people who worked on a game is nearly arbitrary once your publisher enlists a QA contractor or starts localizing to more languages. I don't think it's ever been murkier territory to try to classify a game as indie.