Article mentions that there are indications this was in the wild as far back as iOS 7, suggesting this isn't directly linked to that Zerodium bounty.

The Article mentions that the exploit has kernel mappings going as far as iOS7. This doesn't mean this predates the bounty at all, the bug that received the bounty payout for all we know might have been simply functional on iOS 7-9 or even earlier (and who ever made the final commercial product just didn't bother). iOS7/8 is most likely still used since older iPhones stop receiving updates at some point and older iPhones are the ones you might actually find in emerging markets and developing countries. While rare you can still see people even in "developed" countries running Iphone 4's, if you go to the middle east, africa, or asia you probably see considerably more of them through being sold on the secondary markets.

Older iPhones become the "kids" phone when daddy buys the new one. There are more of them out there then you think.

I guess so, but it's rare to see iPhone 4's at this point when the iPhone 7 is almost out of the door.

Also depending on how old the kids are it might actually work in reverse =)

You're right, missed that. Still possible the Zerodium exploit uses the same vulnerabilities.