pretty impressive, but sort of makes sense too.
He has had some experience with track in the past. He's mentioned a few times that since "retirement" he's actually gotten quite a bit stronger on shorter / punchier climbs. (Which again, makes sense when you don't need to kill yourself daily) quite a few years of big base to pull from as well.
It's not "that hard" to take a guy with big power and make him ~ .2 cda. Once you nail the w/cda, it's just getting down the technical / going full nerd on the aero gains, and then putting in the specific training. If he's doing 4:30 on a "lark" he's not too far off.