arya betrays him and holds her new dagger to his neck saying "i told you not to trust me" before opening his throat
That wouldn't make sense cause Arya wasn't even there when that happened
I think he means if Littlefinger makes some sort of deal with Arya and she says "you shouldn't trust me", then later on she breaks the deal and says the same line. Basically if Arya does to Littlefinger what he did to her father, and the line was a coincidence.
Ehhh there's already been too many cutesy little nods like that this season. ---