The seemingly very intuitive arguments in Bell's book "a primer of infinitesimal analysis" are only heuristics.

This kinds of kill the whole idea. The book is written as if the intuitive arguments were actually rigorous; what's heuristic about them?

I mean the whole point of SIA is that you really can have a triangle where one side is infinitesimal, because infinitesimals are part of the number system, it's not something extra.