Yeah, something about volcanos. He didnt say that it isn’t true, but that he can’t prove it to be true.
Years ago I came across a theory that the "pillar of fire by night" and "pillar of smoke by day" was the result of a volcano. In the day the smoke was visible and in the night the erupting volcano would look like a pillar of fire.
It's an interesting theory but it's still just a theory. To start from the theory and conclude, even based on the most solid evidence of no volcanic eruptions around the time of the exodus (to be clear, I have no idea whether or not volcanoes were erupting at that time), that the exodus never happened, is to put too much credence in the theory.
It's reasonable to look at a description of something in Scripture and try to figure out what it might have been. It's also reasonable to figure that what we come up with is probably little more than speculation, unless we happen to stumble on something that definitely ticks enough boxes we can confirm something. It seems to be the kind of exercise when we might come up with something concrete that provides support that a story is probably true but is unlikely to come up with anything that provides support that a story is probably false. It's the whole issue that absence of evidence is not evidence of absence.
ETA: In this post you say "he didn't say it isn't true" but in your first post you said "he came home with the belief that (the exodus didn't happen)". The two are very different propositions - there's a huge difference between "I can't prove this is true" and "I believe this to be false".
