Regardless of how the labels are used, there's a distinction between lacking theistic beliefs in some narrow, technical sense, and lacking all religious behaviours and attitudes (including ritual, ceremony, and faith in myths and mass delusions).

"Atheism" can be defined as the former, because it's the rejection of theism (the belief in gods and the supernatural), not necessarily the rejection of religion in the wide, sociological sense. "Godlessness" would mean the lack of theism and of godliness in that wider, behavioural sense.

But again, my point isn't semantic. It's about the conceptual difference, not about the labels.