Share uses the same discussion content model as Explorer, but in a much simpler "Web 2.0" style - i.e. a single top-level topic. It was felt this simpler approach is what the vast majority of users would find most usable. There are issues around this approach, as you've spotted, also meaning Share can't see version-specific comments. The commenting service is in need of a refactor for various reasons, the above being some of them, but I don't have timescales for when that might happen. I suspect we wouldn't support the full Explorer discussion feature however.
Thanks,
Mike