That is a very good question and I really like the romanticism of this conversation and especially this sentence. I think if Kay would have gone straight to Michael and told him what she had heard maybe Tom would have been in real trouble (let's say killed, I do not think it matters) but I think Tom basically took no risk, he knew that Kay would not tell Michael this directly. On the other hand Michael probably had an idea what happened between Tom and Kay (he was smart and very receptive for changes in people's behavior)but was ok with it until nobody talks about it.

For one thing I was always wondering why Tom hesitates to tell this "final truth" though, he has to tell Kay somehow that this conversation never happened.