I'm not sure where you're hearing this, but everything I've heard has said they are going to play Kirk outside. I've heard it talked about on local radio, and seen may articles about it. Here's one:
https://247sports.com/college/texas-am/Article/Kirk-was-a-slot-receiver-for-the-Aggies-but-hell-moving-outside-in-the-NFL-117782530
"Kirk said during his brief draft call his offensive role didn’t come up, but he said in a pre-draft meeting the Cardinals noted that Fitzgerald plays the slot so Kirk would most likely play outside – although he is ready to do both."
Just because he played a lot of slot in college doesn't mean that's all he's going to do in the NFL.
Again, as Chaka said, I'm not a huge fan of the Cards' WR 2 for this year, but I'll take the over on 29 catches for Kirk pretty darn easily.