Bissegger must be extremely aero, because his incredible TT performances have seldom translated to road races, especially when they involved some climbing (the lone exception being his win from the break in Tour de Suisse).
All the other riders you mention must have had a bad day or are far from their peak, because the power output required to be in the leading group up to the final 500 metres of the stage is nothing to write home about for a WT rider. I'd even argue this wasn't Ganna's best climbing performance of the last couple of weeks if you ask me.
Maybe the weather, heat and wind, did indeed take its toll on all of them... I know, I am really bad with numbers to begin with, but in addition I feel it's always hard to compare numbers because we don't know the real feel of the race, so I look at other riders on the day a lot. Maybe Ganna is simply pretty good in, for instance, heat, in comparison, that could be, or maybe he dealt better with the wind (more likely with his stature). Plapp of course also did pretty well. What does that tell me? I'm not sure.
Of course there have been quite a few surprises already, especially among the teams I would have expected way weaker, like Arkea and Trek. I don't know. Well, you all know I'm no expert. From my intuition I can only say I was astounded.