I think that they operate on the assumption that the system zoneinfo is "right" and contains leap second information by itself (and "right" is not a pun, tzdata

I think that they operate on the assumption that the system zoneinfo is "right" and contains leap second information by itself (and "right" is not a pun, tzdata does provide Right/* zones that exactly do this!). And I agree to you; this is a very fragile assumption even when you have a total control over the system.