They are considered a very low risk and can borrow for a long time at low rates. They recently issued a 100 year bond.
They seem to have decided to issue equity rather than borrow more. This is probably so that they can maintain the ability to borrow very cheaply in future if necessary.
Issuing new equity might be a financial engineering experiment. No other mag7 has tried it. Plus they got BH name on the plate.
So I guess Google doesn’t think their stock is particularly cheap, but Berkshire Hathaway wants to buy more anyway. (At a slight discount.)