The quoted WSB post expressly says "Corner the market." I'm no lawyer but I'm pretty sure cornering the market counts as manipulation and is illegal. Moreover, planning it out with other colluders sounds like conspiracy.
Cornering the market is bad because the cornerer can literally ask any price. Short sellers (and options writers) are contractually obligated to buy shares, but they can't pay infinity to the cornerer so they go bankrupt. Yay the WSB crowd put the evildoers out of business!
But wait, the short sellers owe people money (technically shares). They had borrowed the shares from someone, and that someone will not get the shares back. Same with options writers: if the buyer exercises the option then the writer has to provide the shares, which cost infinity. So the options writer goes bankrupt and the buyer doesn't get his shares. The call option he bought is actually worth nothing.
In reality a clearinghouse protects the longs from counterparty risk (don't get me started on brokers). But clearinghouses do not have infinite money to pay them off, which is what is required for a stock whose price is infinity. So the clearinghouse goes under or, more likely, Federal regulators step in. But the Feds can't buy a stock at $infinity either. So they say sorry, you option buyers and stock lenders are out of luck. So a bunch of people with stock or options worth infinity dollars suddenly have zero.
Now that all the short sellers and options market makers are gone, some brokers have gone under and the Feds have backstopped the clearinghouse, and a bunch of the longs are left with zero, we still have some longs who own a bunch of infinitely-valued stock. The contractual buyers are gone, and nobody can afford the stock. You can't eat a stock, or buy a car with it. So the selling begins. The price collapses back towards some sane estimate of discounted future cash flows.
The point is that cornering a market is bad for the financial system, so brokers, clearinghouses, and regulators will (and should) try to stop it. I'm sure some of you will say f*** the financial system, and all I can say is enjoy trying to barter with the neanderthals in the cave next door.