Claiming Vingegaard was a nobody before 2021 is misinformation.
He had a number of impressive performances since very young age. His 9th place in the MTT to Balea Lac at 19, for example, was an indicator of climbing talent. And the competition wasn't as bad as one might think: a few sputniks, Rebellin, Pellizotti and Bernal (who's only a month younger than him). Crashes slowed down his development but he still had a few relevant results among the u23 ranks, he was very consistent in 2018 up until he crashed out of Val d'Aosta.
After turning pro with Jumbo, he showed potential in a few races here and there. His stage win in Pologne came against tough opposition and he was good throughout summer after that race.
The problem is he rarely showed consistency and surely not the kind of day to day climbing level required to contend for a GT. The day after his stage win in Pologne, while in the yellow jersey, he lost almost 15 minutes (and not for lack of team support).
Even in the 2020 Vuelta, a race which according to someone proved his ability on long climbs, he had a few very promising performances (Angliru above all) but he completely disappeared in other mountain stages: he lost 17 minutes in the Formigal stage where Roglic was left with no support and lost the jersey and more than 20 minutes in the Covatilla stage where Roglic was again in danger.
Up to the 2021 UAE Tour, where he won the Jabel Jais stage but still didn't feature in GC, I viewed him as a potential super domestique and elite stage hunter. Best case scenario to me was winning GT mountain stages from the break and possibly some one week stage races. But something was clearly changing. Coppi e Bartali was an indicator although against lesser opposition, Itzulia a confirmation: he was climbing with the top talents in the sport on a day to day basis.
@Netserk has a point when he says there wasn't a gigantic leap from Itzulia 2021 to Tour 2021, but prior to that the leap was indeed
gigantic. And he keeps improving: he's now climbing a lot better than the top talents in the sport, while having the best recovery and possibly winning TTs at the Tour de France.