I still think it's rather funny that a guy who couldn't follow the past few years, is saying how much faster they are riding this year... yet he has no issues following now. And we're supposed to take this as gospel?
Fresher - Top contenders go to great pains every year to optimize their Tour prep. Have they all been overtraining all these years, even those with limited calendars? I doubt it, but it's possible. A reasonable question but I don't see any evidence for it. Many riders rather seem to be coming into the Tour (or not even qualifying) undercooked and dropping.
There is no evidence for it. Neither is there for any of your other points. It's all speculation. I think that's why he's asking.
The days of Armstrong, where GC riders would ride only a few racedays before the Tour, and close to nothing after, are long gone. Many GC guys try to ride 2 GT's. Many will try to ride the WCC or Liège/Lombardia/Fleche/Amstel. Which means in the current age, they either have to peak multiple times, or they have to keep their peak form for much longer. Not only the GC guys matter, but also the domestiques, who often ride a classic season in the spring. This year however, they all came to the Tour with only a limited number of race days.
Unlike in past decades, it's a lot easier for riders to train into shape with powermeters, heartrate monitors, computer software etc. I'm not saying it's ideal, but it's not like in the 90s when riders had no idea where they were at, until they entered a race. For instance, van Aert (as an example) who has been critical for Jumbo, came to the Tour after Strade, San Remo & Dauphiné. Last year, he did a full spring classics, (Strade, San Remo, E3, GW, Ronde, Roubaix, Amstel...) which only started about a month after his last CX race, before going to the Dauphiné and the Tour. No, there is no "proof", but i think it's only a logical conclusion, that not only the GC guys, but also their doms are a lot fresher than other years. And i do believe that the disadvantage of not having a normal prep is for the most part offset by the advantage of modern tools and information and is outweighed by the entire peloton being a lot fresher. This year, most of them will try to peak once, and make the best of it during that time, and the TDF is at the center of it.
But no, there is no proof. Neither is there proof that they're all doping.
There is also very likely more quality among the top contenders this year compared to last year, when Alaphilippe finished 5th and Kruijswijk on the podium.