Of course he found out that he can increase the price of a dogecoin if he tweeted, so the second time I'm sure he already invested in Doge and tweeting it again to make profit easily. I don't think Elon Musk was just tweeting about doge with no reason for sure he invested it.
I think this is just a distraction.
While everyone was thinking about the DOGE, he bought Bitcoin.
If he wrote about Bitcoin, he would have to pay a lot more for it.