If a broker that handles a very large portion of retail trades suddenly blocks purchasing a security that retail trading has driven the price up then it stops retail pressure and the stock price will obviously drop.... you could say Robinhood cost retail money by removing the upward pressure on the stock. The SEC report on the incident mentions the price rise was mostly due to retail FOMO, and not the suspected short squeeze that got the early folks attention.

A laymen would look at what Robinhood did and think Robinhood cut the fuse to the short squeeze bomb.