I don’t think your friend is that much of an outlier. Being a TTer and therefore having a good understanding of how to maximise speed at any given wattage is likely as big a part of the solution as his body shape. I’m quite a bit bigger than him at 6’1" and currently around 80kg and I routinely average around 20mph on ~200W on solo rides with a moderate amount of climbing. And Strava power normally shows up as quite a bit lower than Garmin power. E.g. I did 100 miles solo with 3000 foot of climbing at exactly 20mph the other week, according to Strava I averaged 187W, Garmin had me at 205W, I assume Strava includes stops and/or coasting in their number.
I do quite a few TTs, including road bike TTs, so my position is fairly aero and I can put out power while staying aero. I also have a pretty good feel for where it’s worth putting out a few extra watts to get some more speed vs where to ease up and recover. All my kit fits snugly. I have an aero road helmet. I run 45mm deep wheels with decent tires even on my training bike. I shave my legs. This stuff all adds up to some pretty decent speed differences when taken together. And amongst the guys I ride with I’d say I’m average - there are some who spend far more time than me optimising the aeroness of their setup, either because they’re competing in road bike TTs and/or because they’re aiming to win races from solo or small breaks.