As noted in the article, the plane had been modified with far more than just the STOL kit.

> A further issue was that his Cessna 185 had been extensively modified. The addition of floats, cargo pack, short take-off and landing kit and a three-blade propeller had never had their combined effect documented.

That’s a lot of things that modify the flight characteristics of a plane, all interacting together in what seems to be a previously untested configuration.

I can completely see how each individual modification might modify the planes flight characteristics in a well know manner. But I struggle to see how anyone could realistically predict the result of all the modifications without some basic empirical testing.