Well, Nas does say 'pour a little cherry wine' or something along those lines... It's pretty unclear to me whether this track was intended to be a part of her new album since she tweeted the lyrics along with all the other hints about her new songs, or was it a collaboration which they were both aware of... If the latter was the case, I'd kind of buy the double meaning theory. If it was meant to be HER song, then I'm 100% sure she was singing about her guitar because it just makes sense, like @Mrs. Jones said. Although I still don't understand if this was a Frank-era track or a new song, because of the tweet. It does sound like a much younger Amy to me as well, but I find it strange that she'd include such an old track as part of her new work.
I always thought the 'wine' in Cherry Wine was a little twist her and Nas did, in reference to Winehouse.
The way I see it, the two of them wanted to work together but at the time she couldn't/wasn't writing any new material. Nas' schedule was tight (I don't think he was in London very long when they were photographed in 2009), so I think they both agreed to use some previously recorded material and probably chose it together, him saying he could rap over that, maybe even writing some of the lyrics while he was with her and completing it after her death. They had obviously already agreed on the title, and that's why she tweeted it (even though it wasn't complet yet). Also, the fact that he said they used to skype quite a bit makes me believe that their studio collaboration was rather limited, so it was just easier for her to give him something she had already recorded for him to work on....thta's my theory anyway, it could be 100% fictional...
But I'm pretty sure this was recorded before B2B. It's not just her voice, but the lyrics. To me, those are clearly the writings of a young fresh naive Amy, who hasn't even known the pain of heartbreak (certainly not Blake's, and maybe not even Chris'). "Where is he, the man who is just like me"?... I feel that these are the thoughts of a teenager, who hasn't got much else to do in life but think about finding her dream guy, go home listen to records and play her guitar. If I ventured a guess, I would say this song was written when she was about 17-18, and recorded during the Frank sessions.