Pure unlimited tolerance would include tolerating someone’s breach of contract, logically speaking.

That “pure, unlimited tolerance” is what they mean by tolerance as a moral standard. Tolerance as a contract is “we have each entered into an agreement to be tolerant of each other. If you are not tolerant of me, you have broken the terms of our agreement, so I will not be tolerant of you.”

I don’t see a slippery slope here; I’d be interested to hear more about why this is a dangerous road to go down.