I thought he talked about giving money not buying shares. Any gift at all can be thought as an investment in the broader sense of the word so beloved of governments spending taxpayers' money.
I think what underlies Page's thoughts is that he thinks Musk is more effective than most charity. He sees charity as a sink that will provide no long term improvement of the human condition. Honestly, it's hard to disagree.
Except for Bill Gates, precisely. His fight against Malaria has been tremendously successful. Would Page object giving his money to Bill Gates? I don't know, I hope he wouldn't.
Well, Bill Gates seems to want to fight humanity's problems of today - AIDS, Malaria, Poverty. Elon/Larry want to go to Mars and build vacuum-tube trains from SF to LA. One is a real need, one is Science-Fiction. Obviously the latter is more 'fun'/nerdy.
We call that investing.