Even from the first time I saw TWOK I figured Carol Marcus was the "little blonde lab technician" Gary Mitchell "aimed" at Kirk. While lying in sickbay in "Where No Man Has Gone Before" Mitchell boasts that he outlined her whole campaign for her. Kirk sputters, "I almost married her!"
I thought the same thing. And as we know that Nicholas Meyer scoured the entirety of TOS to find an appropriate premise for his movie, it's likely he latched onto that nugget of information about Kirk's past and used it to create Carol and David's characters.
One thing that surprises me is that Mitchel's name is never mentioned again in any TOS installment, be it TV or film. He's just gone.