The letter didn’t restore MJ’s memories,it gave her something almost more complicated: evidence. She now knows Peter Parker existed, that she apparently loved him, and that some supernatural event erased him from her memory. But knowing something intellectually isn’t the same as remembering it or feeling those emotions. So when MJ says “I don’t love you,” it still makes sense because she literally can’t access the memories or feelings attached to that relationship. But the letter plants a seed: she now knows there’s a missing chapter of her life, and she has a name, a face, and Peter’s own words attached to it. The letter didn’t give MJ Peter back….it gave her a reason to start looking for the Peter she lost.