That's simply not true? Most of the dominant Italian fall seasons are off the back of Vuelta participations. Between 2012 and 2018 all Lombardia winners raced the Vuelta. Yes, Pogacar is an outlier, but he's Pogacar and he only really wakes up for Lombardia. Enric Mas meanwhile suddenly turned into a 1 day racer off the back of the Vuelta, and even Landa went from anonymous Vuelta to Lombardia podium with basically nothing in between. Being good in Lombardia is actually harder if you don't race the Vuelta.
I also really doubt Lombardia would take any sort of priority in planning Evenepoels season. I think they just decided first and foremost he would only race 1 GT, picked the Giro, and then filled the rest of the season.
The one thing about Pogacar and Evenepoel will be that they won't be pressed that easily into doing 2 GTs a season to save their season or whatever.