It looks more like Elon Musk bought Dogecoin long ago, and now decided to raise the price by writing about it on Twitter. In any case, it's good that such famous people are promoting cryptocurrency. Despite his "weird" reputation, many people follow him anyway.
This is all under speculation, there is no proof that Elon did buy or at least own Dogecoin, and it is obvious that he is just shilling for it prior to the full bitcoin disclosure. So for me he uses Dogecoin first to see how he will influence the price, and then gets into bitcoin and continue to shill for it, win win for him and for us bitcoin enthusiast.