Market manipulation requires intent to artificially change the stock price. It isn't everything that changes the stock price otherwise any large organisation or person with clout would be constantly market manipulating all day long.
RH CEO’s denial said they made the move to protect the firm and their customers, but blocking buying and selling would’ve given them the protections without the risk of causing a change in the price. So it’s hard to believe that RH’s intent was anything but pushing the price of GME down to cover their customer’s short position.
Again, it's fine to do things that cause stock prices to move. That robin hood could have reduced their functionality further in order to affect the stock price less is not relevant. If they have some duty to go out of their way to minimise their impact on stock prices, its above and beyond regular market manipulation.
Adding a static check of holding was likely much more straight forward.
In traditional Robinhood-style, this decision prioritized growth (allowing new users to still sign up and buy)
I guess a different alternative would have been just selling as much GME until they reach the maximum they could make the obligations for, but then it probably would have been considered less "fair" (if one person could buy 1000 shares, and then no one else can buy any).
You may not agree with Robinhood leadership, or Robinhood as a company (I know that I don't), but this can really be thought of as a #hugops moment where they had to figure out how to track down and address the root of these (multiple) issues in real-time. They most definitely did not come out with the "best" solution.