He "rode and carried" it up the mountain (doesn't say what percentage of the trip was riding vs carrying), rode it a short distance (20 meters, the minimum required for the record) at the summit, and rode and carried it back down again.
Also, his sherpa, Nima Kanchha, carried a full-size mountain bike instead of a Brompton and -- of course -- barely gets any credit for it.
According to the article, there was definitely a good amount of carrying but they rode plenty too (why wouldn't you if you brought a bike all that way).
In the photos, there's like 6" of snow. I wonder if there's a video of the "riding" they did. Bromptons aren't known for their ability to traverse deep snow!