Did the climber keep his sleeping bag in a dry bag? Was his Lithium bag saturated before it was thrown into a dry bag? If so, then of course it froze, they were basecamped in sub-zero temperatures in the Alaska Range.
I watched the video of this climb and it was successful and nothing was stated about his frozen bag during their climb, which would have surely ended their summit push. They had to ski back to basecamp 6 miles pulling a pulk sled full of their spent climbing gear through a seriously fresh layer of snow after bagging the 11,300' Peak. His bag isn't necessarily frozen solid like a brick due to moisture transfer in the night from wet synthetic clothing, it's frozen solid from being saturated and thrown into a stuff sack without being adequately dried out. Which like you said, is hard to do after multiple days of climbing in conditions like that. If your damp synthetic TNF Cats Meow bag was thrown into a stuff sack in the bottom of your pack and dragged through snow for hours in sub-zero temperatures it would freeze up like a brick much the same. Down or synthetic, doesn't matter, saturated bags exposed to sub-zero temps will freeze up. Keep your bags dry.