Roglic used to be an international level athlete in ski jumping, a sport where force is generated very, very quickly. Okay, he was not a top dog, but one would presume that to reach the level he did would require at least some favourable genetics (fast twitch predominance, excellent coordination etc). At face value, force generation in ski jumping is an instant burst, ie. almost exactly the opposite of that in endurance sports and especially time trialling requiring a relatively long, relatively constant effort. GTs too, of course.
I have seen some accounts that attribute his quick rise to his background, especially to his ability to generate pedal torque with the posterior chain (glutes, hams, lower back). Maybe he is very good at this, both in absolute and comparative terms (vis-a-vis competitors), thanks to the training he did as a ski jumper. The implication that he might have some kind of neuromuscular advantages relative to those who have ridden from a toddler sounds worth pondering to me. Research tends to favour the idea that heavy strength training is very beneficial to endurance athletes.
But obviously none of these possibilities, if even remotely accurate, address the question of enhancement via doping.
All said, I am genuinely surprised how smoothly he has switched over to predominantly aerobic, slow twitch sport. And not only switched over but become a serious GT threat. Again, a combination of 80ml/kg/min ish vo2max, high fractional utilisation at threshold (45min-ish efforts) and high efficiency are basically an entrance requirement to the level he is at, and this too amounts to a win in the genetic lottery.
So the guy won the lottery twice? Or did not actually win in qualitites required in ski jumping, and thus switched?