I agree on your last comment, nevertheless, every ferrate as a whole typically has an overall scale. In other words, tags on individual ways classify the "sub scales" (which you typically find a top), while the scale on the entire ferrata classifies the ferrata in its entirety. This is also common practice for other relations like cross country tracks for example.

Anyway, I would kindly invite you to add to the discussion in the referenced wiki-page, this "discussion" here will eventually get lost.