Buffett is buying treasuries because that’s the best way to reap income from his large pile of cash in a safe manner while waiting for the market valuations to decrease.
If treasury yields keep going up, Buffett will probably buy more treasuries and enjoy the high income yield on his investment. If treasury yields decline, if he sees great opportunities in the markets, he will sell his treasuries for a gain and buy the dip on stocks with the proceeds.
Ackman is shorting treasuries because he is a trader and sees shorting treasuries as an attractive opportunity.
Both have different objectives with their money. Both are approaching the macro environment in a rational way. Both individuals will come out of the crisis wealthier than today.