Riding at ~1000Kj’s per hour is ~300w, which is Z2 for him. And while you are burning glycogen riding in Z2, the majority of the work is being fueled by fat (which is essentially unlimited). Even if 1/3 of the work is being fueled by glycogen (very doubtful, even at altitude), he could absolutely eat enough to keep up with that amount of burn. There were certainly some hard efforts well above 300w early in the race that tapped deeper into glycogen stores, but he still must have been keeping up by fueling well.
All that said, it’s not as simple as just having available glycogen on board. Having the fitness/physiology to tap into that after putting 5000-6000Kj’s through your legs is impressive.
I recently listened to a podcast that talks about how ATP is produced (ATP basically being the fuel that produces work). They do a nice job of stepping through the differences between producing ATP via sugar vs. fat, but in a very simplified fashion. Even with the dumbed down version, my biggest take away was that there are a lot of moving parts that have to line up before fat becomes ATP (again, not as simple as just having the glucose on board).