Well......
In theory most event exclusives should be available without ribbons, therefore blocking people from requesting Mew and the like outright defeats the purpose of blocking the ribbons (which really also prevent the trading of events that had special moves, etc).
Mew and Deoxys both had events in Gen III where they could be caught in the wild (Mew was only in Japan).
Celebi and Jirachi were a preorder bonus for Colloseum (celebi being exvlusive to Japan)
Gen IV saw Darkrai and Shaymin both events that uou could catch in game. Manaphy was an egg event via the Ranger games.
Gen V saw Victini the same way, and saw Arceus as a Dreamworld Event.
So far only Keldeo, Meloetta, and Genesect are only available with a ribbon that would preventball trafing.
I tried trading with someone offering Darkrai, and failed to trade it regardless because it was a special Pokemon. I think he's trying to say that there's no point in those special Pokemon being able to make the GTS "list," if you call it, when they're not even tradeable in the first place.