I don't know about violins and cellos, but when I bring my Gibson Hummingbird acoustic guitar out in the cold it'll definitely go out of tune. ...but then all I have to do is re-tune and I'm good to go. If violins/cellos are similar, Yitzhak and Yo Yo would've had plenty of time to tune up to the weather conditions.
It’s not only the tuning. Playing a multi million $$$ Stradivarius violin or cello in sub freezing temps could destroy the instrument. It’s also pretty difficult to play with numb fingers.
The piece was recorded the previous day by all those who actually performed it. This is much ado about nothing.