I predict they will have bought $20bn of Alphabet since the end of Q2. That might earn a lot more than buying Berkshire stock at a middling valuation level.
The reasoning is simple:
(a) they were pretty big buyers at $350
(b) it has been spending a lot of time below that lately, current price $327.50
(c) Berkshire doesn't usually dabble, once they find something big that meets all their criteria

As a lesser speculation to add to my speculation above:

If the soon to be released 13F reports large Alphabet purchases in Q2, which will necessarily have been at average prices above the current price of $288, that might conceivably prompt a rally in Alphabet stock. Alphabet is gigantic, but it's surprising how many people pay attention to moves attributed to Mr Buffett.

Of course the price at $288 is still seemingly quite rich compared to what has been typical in the last decade (at ~8.5 times trailing sales versus a typical number closer to 7), so it's not like a buyer today would be bottom fishing. At least 8.5 is better than 11-12 in the spring, or the 10+ that Berkshire paid in Q2.

Better a great company at a moderately rich price than a poor company at a great price? Especially if it can soak up billions.

Jim
I guess this post makes me a speculator?