I think where the dev gets into trouble here is that he must have a Slack account in order to have built this. That means that he himself has agreed to the T&Cs at some point. If he could have built this without having an account, then he would have a better chance of being able to avoid liability.

In that case, there would be no "contractual privity" between Slack and the dev, and Slack would have to go after him for inducing breach of contract or some such thing.