I think you are missing the point. It's not about being on a super peak of form for the targeted day, in which classics champions reach a level of fitness that may be equal to or greater than a Tour champion, if he happens to be competing in the race. But being able to hold a super peak of form for 21 stages, over the high mountains and tts, without having a really bad day.It simply doesn't make sense to say they are on a different fitness level. If they were, all the one-day races would just be theirs for the taking.
So obviously being a Tour champion doesn't gaurantee just bagging all the one-day races, although in the past Tour champions like Merckx and Hinault especially could bag quite a few monuments (and win grand tours). But classics specialists could not do likewise in winning grand tours. That's because peaking for one day is one thing, maintaining an increadibly high level of fitness, without a major decline, over 21 days is entirely another. I find this to be the critical and decisive issue.
With the general higher level of fitness in modern cycling, however, the type of effort to excel in both the one-day races and the grand tours has lead to intensive specialization, because the one-day peaks of super fitness are higher, as is the level of form that's maintained for three weeks in the grand tours. The result has been that the one-day specialists and the grand tour specialists rarely ecounter eachother in the classics' seasons in modern cycling. Hopefully though this trend is changing. And the teams working with potential Tour contenders have been saying for years that to place high on gc there means total dedication to that goal, 12 months a year, otherwise failure is assured