It reads to me as though speaking in Congress can't be made a felony (or treason, or breach of the peace) because that would involve the Member being questioned

It reads to me as though speaking in Congress can't be made a felony (or treason, or breach of the peace) because that would involve the Member being questioned for their speech in another place (i.e., a court) — but then, this is the sort of thing that makes American politics such fun to watch, from a bit of a distance.