I'm not sure if it's got anything to do with custom tactics, as someone said above, but I did hear rumors that it was supposedly possible to use a custom squad (your offline squad actually) in UN-ranked matches. I've actually heard of some glitch that allows this to be done in ranked matches but I've never experienced that. I checked this out though and it does work. I myself put Beckham on Milan and played a friend of mine in an unranked 1 v 1 and he was in fact on the team. I changed his number to "75" and everything. It appears that any changes that you make to your regular offline squad file, are actually kept when you go to play an unranked match. I put the gloves on Berbatov, Evra, and Tevez and all of this registered in the online unranked match.
Now, if the guy that played the super team was playing a ranked match, I don't know how EA let that one slip by. Maybe it's got something to do with them being kind of incompetent.