According to the Metroid timeline released by Nintendo, Other M is the fifth time she fights with Ridley. Why would in the world would she have PSTD when seeing Ridley when she "killed" him four times? By this point she should be annoyed the fact that he can't seem to die. The PSTD was stupid and contrived.
Then again, Other M seems to disregard a majority of the stuff that happened before, so what do i know?
I don't think that's how it works. PTSD, similar to most anxiety disorders is not so easily treatable with exposure. The effect Ridley would have on her if he indeed killed her parents in front of her would be so hard wired that she would probably experience that same level of fear with every Ridley encounter. You just didn't see it before because this is the first time we get to experience what it is like being Samus.
Granted, most people are calling bull on Other M's perspective of the Metroid lore, so there's no sense of me in defending it years after it's released, but I feel it being completely reasonable for her to have a deep routed fear and hatred towards Ridley.