Recursion depth limit is really a memory limit, so you just need someone to come and install more RAM when you run out in order to obtain de facto infinite memory, hence infinite recursion depth.

But this is not a de facto recursion limit of the system - the limit is encoded in the program itself, in the macros used to implement the limited recursion.

void f() { f(); } is only limited by the system, and if compiled on a system that automatically expands the stack to fill all available memory, you could in theory keep adding memory, and the system could handle more recursions. The recursion limit is external to the program.

But the linked c-preprocessor programs would have to be explicitly modified by adding more lines of code to achieve the same thing.

Yes, but as I point out elsewhere, Turing Completeness separates the expression from the machine. Machines will always have limits. But the expression of computation does not have to have limits.

How does tail-recursion fit into this discussion of infinite recursion?