At best, that is the symptom and not the cause. They wouldn't guarantee him the job because ________________________________. If everything had been hunky dory, they would have said: "You have nothing to worry about, Tom." But they didn't, and there has to be a reason for that. And that reason(s) is why he left. There is something that they saw or didn't see in him that made them say: "We can't make any promises", or worse yet: "We really don't see this working out for you, Tom." It wasn't that conversation that made him leave. It was the reasoning behind why they couldn't promise him the job that made him leave.
By way of example, a person who works under you botches three projects in a row. That person comes into your office and says: "I hear that the company will be promoting someone from my Unit. I would like to be considered for that promotion. Do you think I have a chance?" You say: "I don't see you getting that promotion." The employee then resigns. Did they resign because of the conversation, or was their departure the result of their poor performance that then caused you to tell them that the promotion was not in the cards? The conversation is simply the last step of a process that was already in motion.
