Actually he was regarded as pretty much a sure thing when he passed pro.
He won Avenir in convincing fashion, and was the second youngest rider in the top 10. Arensman was the outlier, not the standard. Pogacar won it at 19, a bit under a month before turning 20. Which possibly makes him the third youngest winner of it, french wikipedia only has the 2 youngest, Baronchelli and Gaudu. Gaudu who was born 19 days later in the year than Pogacar. Bernal for comparison is born 8 months earlier.
As for his normal/modest gap, it's the biggest since Quintana in 2010.
So that already was very much out of the ordinary, Add that his edition was regarded as rather strong at the time (now looks crazy strong, but that's in retrospect, at the time with Sosa dominating Burgos, Vlasov, Almeida from the Baby Giro, McNulty I think rated rather highly too, just looked strong)
So his Tour de l'Avenir win, combined with his 2 excellent Tours of Slovenia, 5th and 4th (5th at 18 years and 9 months, others are still juniors at this age) made him special when he passed pro.
He didn't dominate the Juniors? Nobody does except Evenepoel really. And especially GT riders/climbers really don't, Junior results have never been a good indicator for success in GTs. And let's not forget that Pogacar was born in September, not in January.
He was a sure thing. Like Bernal. But of course not every sure thing pans out, see Sosa and Sivakov. Too many... yes, strange times (add Ayuso too now), but before that the last sure thing was Quintana, before that we go back to Ullrich, Franck Vandenbroucke and Santi Blanco. Cycling simply has gone crazy the last few years. Of course nobody in his right mind thought he would finish third in his first GT then win the Tour twice, while winning 2 monuments and being second in Flanders, plus one of the strongest in MIlano-Sanremo. In 3.5 years as a pro. But he wasn't just one of many, he was regarded as special from the start.