Nah, it didn't add any depth whatsoever. It only added information out of nowhere. This sort of stuff should be revealed smartly via the main characters finding something and talking about something, not by narration out of the blue. That's just bullshit infodumb. You can't really call this Zeref's flashback when the dude hasn't been a character appearing by his own right in any meaningful manner so far. I wouldn't have had anything at all against seeing only Zeref and Acnologia's meeting, that was quite cool indeed, but Zeref's (and Natsu's) early history should have been revealed elsewhere in a different manner.