If Sasha still thinks fondly of him, some weight should be attached to that, I believe.
On the day in question, she had been unhappy after being dumped. Harry was probably unhappy as well - underneath teenage bravado. He was a particularly unscholarly scholar and his mother was dead - or, in his mind, had 'disappeared'.
So Sasha and Harry drank together to have a little transient fun and blot out sorrows. In an uninhibited state, and in need of solace, they went outside for a cigarette break which turned into sex. It felt 'forbidden', which added to the appeal. Afterwards neither of them knew quite what to do or say - they were both very young - so they did not contact each other.
Later on, in Harry's mind, the encounter morphed into the dominant (much) older woman scenario described in his book. Possibly, it was a teenage fantasy that he would replay to himself late at night throughout the lean A-level years at Eton.
My opinion is that he was immature and embarrassed, and did not know how to act with grace towards an old mucker, after a spot of drunken nookie. But that was too painful to remember so his memories 'shifted'.
Sometimes reality is just too real to be palatable.
But in any case, it's clear Harry really can't set himself up as a super hero fighting misinformation, when his own recollections are so dodgy.