No. These are the official names for years approved by the author (for the LN and the manga), the scanlators were just a bit late to change them but at least they did.
And to not confuse:
Demon Lord ==> The Actual Demon
Archdemon ==> Sorcerer that inherited the Sigil of the Demon Lord
If both were called Demon Lord, we'll need to constantly check the context before knowing which one they're talking about.
And the chapter didn't call it 'sigil of the demon lord' but 'sigil of archdemon'
So in your own post there's already a need to check for context, is the 'maou' in the seal's name referring to the people who inherited it (archdemon) or THE demon god/lord itself?
Given the order of things you'd think it'd refers to the demon lord, right? Since the seal was made before the first archdemon came to be and you also call it such.
But that's not what the translator ends up with...