Well, yes, zero can be understood as high-order infinitesimal, especially at the solution ratio limit, and zero can be understood as the highest-order infinitesimal.

You are right, infinitesimal is indeed a variable, and zero is an exact number. But in the process of finding the limit, zero is understood as high-order infinitesimal, which is the end point of infinitesimal.

You can understand it this way: any sequence infinitely close to zero from positive or negative direction, including zero, can be called infinitesimal.

Again, zero is considered to be infinitesimal and is mainly used to solve the proportional limit.