Hard to say if Canc's performance was believeable. He totally destroyed Sagan on the Patenberg and Sagan is such a huge talent. Then again Roelands managed to hold on to Sagan.
I think what happened was when Cancellara went say 90% on the Kwaermont, Sagan went too far into the red in order to keep up with Cancellara,
Because he is a little worse, the same pace took more out of him.
You could see he was already cooked there and almost losing Cancellaras wheel on the flat afterwards.
this didnt effect Roelands as he was in front due to the break so never went into the red to stay with Cancellara.
So going into the Pattenberg Sagan was already weakened from digging too deep on the Kwaermont. When Cancellara put the hammer down, Sagan went into the red again but after a few seconds realized that he was going too far and wasnt going to hold Canc no matter what.
He had gone so far into the red he didnt have the power to solo 2nd and Roelands - who's weakness was balanced out by not being as cooked, caught him and they worked together.
So we can't read too much into the gap Cancellara got in the last 12k because Sagan was more exhausted. + both him and roelands realized the race was over and were hoping to keep the podium.
After the gap went out quickly Canc was no doubt getting info about how Sagan and Roelands had given up so probably didnt go full blast anyway.