Yes, they are very accomplished to do what they did on Gannett (and other notable achievements).
Being a far cry from a "highly accomplished ultrarunner", a partner and I did a one day ascent of Gannett in 2003, sort of. It would be more appropriate to call it a "single push" or "campless" climb. It was a far cry from a "run" and we actually moved fairly easily most of the way.
We left the trailhead at 8 pm, arrived at the base of the climb at 4 am, summitted at 8 am, back at the base at 10 am, hiked until 11 am, and then took a nap for FOUR hours in the warm sun! This was key! Then, we started hiking again and were back at the car by 10 pm, for a round trip time of 26 hours.
We wore trail runners and Kahtoolas for footwear and used trekking poles (one with a BD whippet) all the way to the summit and back.
We did take a canister stove and a Foster's Lager can for a pot to brew tea and cook a hot meals, Aqua Mira to treat water. I took a light wind shirt, Wild Things EP Jacket, windstopper gloves, powerstretch beanie hat, and a pair of GoLite Reed Pants as my extra clothing, and wore Cloudveil rodeo pants and a smartwool shirt.