This guy is only playing with you and the markets. It's all fun for him, he's just trolling.
He was playing with real intention of making money.
This tweet does not only give growth to DOGE but it has also affected other altcoins as Elon is the new hype of the market now.
We don't know if its over or there will be tweet to come soon, but he is really the hero for bullish people since his tweet can move the market resulting into millions or billions in price movement in overall.
He was probably just bored and tweeted something he is interested with and suddenly it got out of hand and it did probably impacted his regular life as well so in return it changed things about him too. Remember this was the guy who "joked" about buying all his shares back for 420 dollars each when the price was a lot lower, and his joke on twitter about this, caused the price of tesla go up a lot during that period, and SEC punished him with billions and billions of dollars just for doing that.
You can just do or say something like that to profit yourself. Of course he never really even said "buy doge you will get rich" exactly, but I am sure SEC will be on him eventually, not right away since it is crypto but there is a chance he would be in trouble if he bought doge before any of this, then promoted it and profited from his own acts like that.