His form was better than expected, but it wasn't 100% either, as the opening ITT showed. While "short" it wasn't a 4k prologue and should have been long enough to do better than frankly about everybody who finished in front of him bar Ganna. The Montalcino stage clearly "broke" him. This was stage 11 and as far as i'm concerned it was a gimmick stage as far as GC is concerned (i don't have anything against it, but you know it can shake up GC and favor certain riders over others, just like a 90k ITT would). He had never ridden over 7 days in a row.
If your base form is not broad enough, there is a point where your body simply can't deliver anymore. And i think that's what happened. He was very strong on stage 6, the only one who was comfortable in Bernal's wheel. And 5 days later he can't get over a hill.
Either he was simply underprepared, or we can jump to conclusions and say he can't do 3 weeks. Knowing he was only cleared to ride the bike, 3 months on the day before the Giro started, and had to start by doing short daily sessions at low intensity, and build his way up to "normal" training blocks over the coming weeks, maybe it makes sense to assume the former without ruling out the possibility of the latter. What happened on stage 16, puts a new light on what happened on stage 14. It means we can't assume a climb like Zoncolan is out of his league, because his form simply wasn't there anymore. And yet, he only lost 2 minutes in stage 11 and 1.5 minutes on Zoncolan, only roughly 30 seconds on Carthy etc. By no means "bad" either.
Any solo or effort is always dependent on specific circumstances. Why would you even think otherwise? Cancellara also wasn't capable of riding away from the peloton at 100k from the finish and win any race he wanted. The question is, how does it help him as a GC rider. Does it make sense to try those solo efforts in a 3 week race, when you have a peloton chasing behind you, with Ganna, Küng and Dennis? That might make sense in a classic, when you don't have to think about tomorrow, and it doesn't matter if you win by 1 second or 1 minute. In a stage race, it will only be useful when the rouleur domestiques have already been dropped, and you only have to deal with lanky climbers who are aerodynamically capped at 42 km/h so to speak. I remember a stage in the 2018 Giro, when Yates attacked and Dumoulin was begging others in the group of favorites to help him, but nobody would. In such cases, Evenepoel's abilties would come in handy, either as attacker or chaser. He could attack or bridge in a valley after the penultimate climb. He could attack in a hard hilly stage where the rouleurs are long gone from the peloton. And he can use his ITT.