Well, Pokebank Celebi does have the Classic Ribbon, therefore it cannot be traded over the GTS. So yeah. :3
As to why the Palkia mentioned in the link was traded, you can't detach ribbons from a Pokemon so I just assume that this person has just misheard is friend since all Palkias that have ever been released have had a ribbon that have prevented it from being traded over the GTS. So it must of simply not have been an event Palkia, or it was just a local trade.
Either way, your Celebi can't be traded over the GTS. However, you can trade it with local wi-fi.
Hope I helped. :)