A broker not executing a trade that requires the broker to spend liquidity, because the broker doesn't have any liquidity is not market manipulation.

It's not market manipulation to not do something that you're not allowed to.

You are getting close to the heart of the matter, though. There was a kind of distributed market manipulation, as is inherent in any crowd mania. Largely coming from the people on reddit screaming 'DIAMOND HANDS' and 'BUY AT ANY PRICE' and 'WE WILL ALL TO THE MOON TOGETHER' and 'BELIEVE IN THE MOVEMENT', and ...[1] while they were quietly unloading their positions to bigger fools.

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[1] My absolute favorite was 'If we just diamond hands Uncle Sam will cover all of Melvin Capital's losses and will turn over ownership of the entirety of the world's economy into the hands of the apes, because contracts, are, like, sacred, and we found the infinite money glitch, and the USGov is somehow responsible for bad trades done by a shitty hedge fund.'

Why are people posting on Reddit a manipulation? Anyone can post anything on Reddit and no serious investor will blindly follow this. On the other hand, companies (brokers) that allow people to trade have a power to manipulate the market by enabling/disabling trade of specific stock at necessary moment.

Also if people posting on Reddit can cause serious losses to greedy capitalists then it is fun.