No, instance A sees no comments/posts by instance B users regardless of if it’s on instance C.

For example, Beehaw defederated my instance.

They could be commenting all through this thread and I’d never know.

Edit: okay technically not here because they defederated LW too.

Edit2: so to be clear, if your instance defederated/blocked dot ml then they wouldn’t be able to interact with your content at all, votes, posts, anything.