I didn't have much of a look at the specifics of his setup - my main point is to just explain to the LetsRun folks that cycling is an only somewhat related proposition to running and it just takes time to iron out all the silly shit that can lead to power loss. My point is that I think this is an impressive initial result, and all the silly criticisms mostly show ignorance about the specifics of biking. I still doubt a newbie could get effective power-based workouts on the high end on rollers. Riding rollers just isn't easy at first.
I tried to run once after 45 minutes of riding and it was hard as hell. And I catch myself making all sort of power-robbing technical mistakes out on rides all the time. You gotta ride outdoors to turn out the most power outdoors.
That "on the hoods" road race breakaway aero position is fairly aero, but still your hip angle is closed up significantly (and riding that position is tiring - I have shorty aero bars on my road bike and it is both more aero and way more comfortable). LetsRunners with no bike familiarity don't get the tradeoffs and specificity of aero/power. There is just all sorts of silly crap you have to iron out on a bike to maximize power that just takes time and isn't a question of fitness, just specificity.
You will see Webb improve rapidly on the bike. Just takes time.