I suspect the first step is trying to match like cover art. Then it might compare several seconds of the song it's working on. I don't think Apple is going to say exactly how the matching stuff works.
In any case, having seen mistakes from cover art, I hope this is better at not making false matches.
I do know I have many albums that are not available from iTunes, so it would be a few days at least probably to upload everything that it couldn't match.