Buffet's said he was unhappy that apps like Robinhood let regular folks trade without paying a fee. Well, golly gee! That sounds terrible. I guess he thinks trading should only be for people like him who can afford to pay big fees.
He also doesn't like that everyone can participate in the "gambling" aspect of the stock market -- something from which he's profited greatly.
tldr: Robinhood got offended that Warren Buffett and Charlie Munger called their company out for incentivizing gambling and not investing... which seems particularly obtuse since Robinhood makes their profit from the r/WSB community and not the people who use their platform for long term investments.