Hahah..."that promise to honor them". There's no such thing. These exchanges can announce that they want to honor some forking coin but if there is no market for it there's nothing they're not going to want to spend the money to create the infrastructure to support it! These aren't charities!
I remember when BCH forked nobody could deposit or withdrawal coins due to the slow network yet people who had BTC on Bittrex got coins and could trade the markets right away.
This is because Bittrex decided to make a market for Bitcoin Cash. They decided to buy their customers Bitcoin Cash...the transactions probably weren't even hitting a blockchain, they were merely adjusting the credit for each customer's account record when that customer decided they wanted to trade their Bitcoin Cash. YoBit did the same thing with Bitcoin Gold.