I doubt it?
The remarkable thing about WvA is that he shouldn't be top 20 in a GT in the first place. But top 20 vs winning a Grand Tour is still a different universe of climbing.
Normally, I'd agree 100% ... but he put in a lot of work in the Tour that as a protected GC rider he wouldn't have to do. On the right course with the right support, yeah, I actually think he could be a real contender. He's not going to give anything away in an ITT, and if there's a northern cobbled stage he could take 4-5 minutes on a light climber if the weather's bad (or even if it's not).
Alaphilippe, because of his size, isn't a perfect analogue. But he still finished fifth -- I don't see Wout as a worse GC contender.