It’s a phrase that’s certainly a sweet thing to say to someone, so I’d hope CA fans would enjoy it. It’s typically used as a reason why someone may have feelings for the other person, but like most things concerning Cloud, his mental state should be considered.
I certainly don’t think he means that Aerith brings out the RealCloud. The phrase you’d want to see would be closer to “you make me feel like I can be me/myself/more myself.” Vs what’s said.
Imo it implies that she… well, brings out new sides to him. Himself or not, Cloud would’ve never guessed he’d be willing to crossdress for anything, or have an interest in fields and flowers ( botany in general? ), or photography ( even if that one makes so much sense it hurts ). Like any close relationship, be it friendly or romantic, they change you in little and not so little ways.
Through Aerith, Cloud learns how to treat others gently, and ultimately she teaches Cloud the power of compassion. This isn’t to say Tifa herself isn’t a shining example of compassion, but I’d say Aerith shows how even a simple flower girl can have a massive impact through compassion and acting for the sake of those you love.
So yeah, she brings out parts of him he didn’t know existed, probably because he hyper focused on the parts of himself he lacked instead.