I'm not sure what we're all disagreeing with here. Sure, the collecting medium is different (sports cards vs. comic books) but I don't think it's such a bad analogy. Not perfect but it gets the point across for me.
Sports cards - XRC (cards that were released in Topps year end extended sets and not part of a regular set) and minor league cards = Not true rookie cards (but often the first card to feature a player).
Comic books - Comic characters that appear in a preview book before the true first app (example, the aforementioned Darkhawk appearance in Marvel Age 97) = Not a true first appearance (but the first time the character appeared.)
In both cases, the XRC or minor league cards is not the true rookie card and not the card that collectors focus on when desiring to own the true hobby accepted rookie card.
Similarly, the preview comic that just happens to feature a character before it appears in a sequential comic book is not a true first appearance and not the comic that collectors focus on when desiring to own the true hobby accepted first appearance or first full appearance.
The difference now is that there seems to be a (growing?) fringe element of huckster and opportunist trying to make a buck on the unwary and ignorant by trying to pawn off a preview book as the true first appearance of a character.