Presumably Buffet would respond to you by noting that the stock market is a voting machine in the short run and a weighing machine in the long run. i.e. for the companies he owns, he isn't generally concerned with short term stock price fluctuations.

It's probably hard to buy a billion dollars of any stock without driving the price up, even if you spread purchases out.

For most stocks, but not Apple because it's both very high market cap and very highly traded. It has been averaging 43M shares traded a day which is over 4x the size of Buffet's trade. There are also plenty of days with it trading at much more than this size (it did 113M on April 27).

How many days do you think one would need to spread out a purchase of 1/4 of a stocks average daily volume in order not to noticeably move the price?