I think it was implied that the memory wasn't the cause for Harry being able to see Thestrals; Harry seeing Death in the form of a Dementor was. From Chapter 49:
"They are visible only to those who have seen death and comprehended it, a useful defense against most animal predators. Hm. I suppose that the first time you went in front of the Dementor, your worst memory proved to be the night of your encounter with He-Who-Must-Not-Be-Named?"
Harry nodded grimly. It was the right guess, even if for the wrong reasons. Those who have seen Death...
If comprehending the nature of Dementors is required to see Thestrals, how does Quirrel - or anyone else - see them? Quirrel seems to accept that "seeing Death" means simply seeing someone killed, perhaps someone dear to you.
1buybuydandavis
Thanks, that's a good one.
Ok, so now updating, there is not evidence against the memory being implanted. Also, that fact that Quirrell suggests the memory makes it more likely he implanted it.
I think it was implied that the memory wasn't the cause for Harry being able to see Thestrals; Harry seeing Death in the form of a Dementor was. From Chapter 49: