"The cross product of those paths with the possible state configurations of all global data (including instance data which, from a method scope, are global) and formal parameters is indeed very large. And the cross product of that number with the possible sequencing of methods within a class is countably infinite."

Sounds more like a condemnation of OOP than of unit testing, and I do genuinely feel sorry for the unit testing, OOP purists out there. I prefer to design more functional methods, which operate on parameters and injected config instead of instance state and/or globals (cringe). Incidently, this approach makes full coverage attainable.

"Large functions for which 80% coverage was impossible were broken down into many small functions for which 80% coverage was trivial. ... Of course, this also meant that functions no longer encapsulated algorithms. It was no longer possible to reason about the execution context of a line of code in terms of the lines that precede and follow it in execution"

I can reason about such an implentation MUCH more effectively by glancing at the small bit of higher level code which integrates everything (and as mentioned above by foresaking instance state and polymorphism). This strikes me as a bit like advocating a flat directory structure cause it's important to be able to see all your files at once.