I agree with the poster: Objectively (and absent any outside knowledge for context) the quoted headline invites the interpretation that Minnesota already is a playoff-caliber team and that Adrian Peterson thinks Michael Vick is a good enough player to make (that is, not get cut from) the squad.
As for the argument that the headline writer was constrained either by (a) space limitations or (b) a rule against indefinite articles at Yahoo Sports, it is clear from the link included in the question that (a) is not the case, since there is plenty of space for the indefinite article in what is already a two-line head, and (b) is not the case, since a quick search of other Yahoo Sports headlines in the vicinity of this one yields "San Francisco 49ers coach Jim Harbaugh does push-ups with a walrus" and "Is AJ McCarron an NFL starting QB?"
But even if space were at a premium in this instance, you could easily construct an accurate headline that is shorter than the one actually used. Here again is the original wording:
Adrian Peterson thinks Michael Vick would make Minnesota playoff team
The entire news item is built on the following Twitter tweet by Adrian Peterson:
@MikeVick would intently make the vikings a playoff team!
Since Peterson didn't merely hint or suggest that Michael Vick would "intently" make Minnesota a playoff team, why does the headline writer find it necessary to use the verb "thinks" instead of the shorter "says"? That change alone would clear enough space to accommodate the missing "a":
Adrian Peterson says Michael Vick would make Minnesota a playoff team
But as long as we're looking for ways to save space, why not replace the verb after "Peterson" with a colon? And why include Michael Vick's first name—has there been another Vick around the NFL since Marcus Vick washed out in 2007? And why say "Minnesota" when you could say "Vikings"? Incorporating those perfectly reasonable alterations would give us this headline:
Adrian Peterson: Vick would make Vikings a playoff team
Or you could flip the wording around:
Vikings would be a playoff team with Vick, Peterson says
The unintended meaning of the original headline would have been easy to avoid if anyone at Yahoo Sports had thought twice about it—but no one did.