Yes, after careful re-read I see my conclusion wasn't 100% accurate, it seems that multiarch is kind of superset of multilib, i.e. one of the multiarch's goals is to solve problem which is already solved by multilib (plus it solves many others like CPU optimized builds, as written there). What seems to be a good thing but as you have mentioned, it seems to be mainly a Debian/Ubuntu's initiative, not FSF's what is shame.