The loss is a paper loss
i.e. the loss hasn't been realized.
BRK plans to, and has the capacity to hold on to their investments while their valuation has dropped.
This is not unusual or unexpected in the current environment. In addition, when you are invested in stocks, you don't loose anything until you sell. I'm sure Mr. Buffett has been at this point before and actually see it as a buying opportunity rather than a negative.
He made bank off the 2008 crisis - literally. Invested in a ton of big banks and made huge returns. He's more than been here before.