I believe the point of the article is that the change in driving dynamics from the original design is what causes the issue, and it's typically more severe with school bus conversions than other types of bus conversions, because of the original design of the school bus and what it's meant to do.

A school bus designed to be offroad capable for carrying children is one thing because it is designed with those purposes in mind. The dynamics of carrying those children, and where the weight is distributed, are taken into consideration in the design process and safety testing. If you use it like it was built to be used, you should be fine.

Once you start changing the distribution of weight, specifically making the load in the bus more top heavy (which is typical in conversions compared to the lower center of gravity when you're distributing the weight of many people through benches), that changes the dynamics in play for the narrower axle of a school bus. Which makes for a higher rollover risk. You wouldn't want to carry 70 kids in a converted bus where their weight is no longer distributed through benches.