I think their point is there's no "peer to peer" trading so it's not an exchange. You're always contracting futures with them, so it's more like a vegetables seller than a trading facility.
Between CBOE and OCC, that’s (exactly?) how options trading works as well, and I don’t think anyone claims that CBOE is not an exchange.
Exactly right. You're able to privately resell contract keys (and the buyer can then re-issue a new key with their delivery info, voiding the old one), but onionfutures.com doesn't facilitate it.