The difference between net worth and having real money is different. Networth is based on what they own in different places like the stock market, real estate, and other things. Imagine Elon sells those shares. The value of the thing will decrease. He won't be able to sell all at once. That's how it works. Net worth is not equal to liquid money. So does he really owns that money or can he have that total in once in order to manipulate the market? From what I can tell, he can't. Or anyone else. It is possible tho as I said, but unlikely to happen.

Also as you mentioned, the more he buys, the more price increases per buy. So it will be tough for anyone to do it unless everyone come together and do it as a group. That won't happen because as the price goes low, others will buy.