Wisconsin 948.60 is their regulation against possession of a dangerous weapon by a person under 18.
Article (3)(c) states "This section applies only to a person under 18 years of age who possesses or is armed with a rifle or a shotgun if the person is in violation of s. 941.28 or is not in compliance with ss. 29.304 and 29.593."
In other words, The regulation against possession of a dangerous weapon under 18 has an exception when you have a rifle or shotgun. You must not be in violation of 941.28 and you can't be out of compliance with 29.304 or 29.593.
941.28 is a regulation on short-barreled rifles and short-barreled shotguns. Kyles rifle was not a short-barreled rifle, so he was not in violation of 941.28.
29.304 pertains to regulations of firearms for individuals under 16 years of age. Kyle is 17, so 29.304 doesn't even apply to him.
Section 29.593 requires anyone born 1973 or later to essentially attend a hunter safety course to go hunting (with any weapon bow, rifle, etc). Kyle was not hunting, so he was in compliance.
So per Wisconsin state law, a person under 18 cannot posses a dangerous weapon, unless that weapon is a rifle or shotgun! If a person posses a rifle or shotgun and is under the age of 18, and over the age of 16, and they are not hunting, section 948.60 does not apply to them per the letter of Wisconsin law! There us no regulation against them having the rifle or shotgun.
Further, Wisconsin's open carry originates from their Constitution and allows anyone firearm that can be legally carried, to be openly carried by anyone legally able to do so. Thus, since Kyle's rifle was a firearm that could be legally owned, and since there was no law against Kyle possessing that rifle, he could legally open carry it.