You're wrong if you think he's "talking up his book" because Buffett has been consistent in what he's said about Apple (publicly) since he started buying it.
He would be just as happy to see it go down because he would buy even more and Apple's own buyback would be more effective (He literally said this in the Berkshire shareholder meeting). He will likely keep adding to his position on any dips, so he has no interest in trying to pump up the price. He's required by law to report his purchases of equities.
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Apple is not even close to what one would call an expensive stock and Buffett is likely not done buying shares. Berkshire still has over $100B in cash to deploy and Buffett has said he'd love to own more Apple. He'd definitely like to buy it lower than current levels, giving him no incentive to "pump" it...lol.
Frankly, it's good for AAPL to go down in the short term. Then Buffett and I can buy more and Apple's buyback will be more effective. Buffett knows this and has said it.
It’s interesting to see Microsoft are now rapidly closing in on Apple (alphabet is still slightly larger though not growing as quickly) - even (so far quite muted) talk of MSFT pipping Apple to the $1T mark... I think half this site would have an aneurism and it would make the other half’s year