I think both is true; the level in general isn't bad. Despite whatever they are saying Movistar continue to either put their priority on the Vuelta or their training plans throughout the year are just still made to top it in the Vuelta, and some guys are in pretty good form. However, there are also many GC riders, and more than for Giro and especially Tour, who don't have this as a priority during the year. In fact, almost nobody from the top guys does. It's very often second choice. But I didn't mean to diminish Roglic's achievements, fact is the Vuelta isn't the top target of the season for him either, and then, when others tend do drop off significantly, he doesn't.
I stand by my assessment that his best isn't as good as Pogacar's best; however, time may prove me wrong.
Carapaz peaked for Vuelta 2020, Carthy peaked for Vuelta 2020, Dan Martin too, while Roglic was seriously fatigued for that race. This year obviously you have Adam Yates who targeted Vuelta. Movistar always targets Vuelta with their best riders, that's why often they're not great during Spring, to have the best form for TDF+Vuelta. And who did he beat in 2019? Valverde who is exact same profile of a rider that doesn't know about bad form, and the ultimate wonderkid Pogster who peaked for that Vuelta as it was his only GT of a year. That's enough top guys on form. Now who are the other riders like Roglic and Piti who are on form even when they're not? MvdP, WvA, Pogi, Sagan, baby Eddy... What do they have in common? They're among the best of the best. I understand your observation which I think is correct, I just don't see how is that a key explanation of Rog's Vuelta "domination".
And speaking about Pog, man has no weakspot, he's almost bulletproof. He has all the strenghts of Rog but none of the weaknesses. IMO he's a better GT rider atm, and Rog was never really top in those 1-day 250+km classics so no wonder Pog beat him in LBL. But you never know what future brings...