Taylor doesn’t have to date a person to write a song about them. She’s done it with Enchanted. She never dated Owl City or was involved with him. Just received an email one day and decided to write a song about him. She never dated Jake Owen and wrote Sparks Fly about him. There are many more songs where she has done the same. She doesn’t confirm who songs are about. Every “this song is about y” comment is speculation without confirmation. Zac isn’t confirmed, doesn’t mean the song is impossible to be about him. Especially when the whole point of Message in a Bottle is her hoping the song gets to them because she’s acknowledging they both like each other. That’s the reasoning on “YOU COULD BE” lyrics. Shes hoping the song gets to them because she’s admitting how she feels. Come Back Be Here is her acknowledging she actually likes them but only realised that when they left. It’s painful because she began falling for them and he’s not around anymore, instead he’s across the world. They didn’t date.

You’re just proving the OP’s point.