A Full Member / Hero account can't (technically) be nuked and by having all its posts deleted, to turn the respective account into a Brand New one?

No, everything should be done manually for higher ranks.

- Some ranks used to be immortal against demotion but that's no longer the case:

IIRC only Newbie, Hero, and Legendary members don't get demoted normally. Maybe I'll fix it at some point, but it's not really a big deal.

e.g. The following account is no longer a "Hero Member":

I also saw a user: mkr with 15 posts and Hero Member (Archive: http://archive.is/ijaTy)

Profile Link : https://bitcointalk.org/index.php?action=profile;u=216331 (Still Hero)

This measure (nuking) applies only to Newbies? And if yes, why?

Correct. It is meant for fighting against spambots:

Nuke removes all of a users posts/threads and bans them (spambots).