I think this is about pre/post increment/decrement being inspired by PDP-7, not about C pointer arithmetics being inspired by PDP-11 ISA addressing modes[1].
https://en.wikipedia.org/wiki/PDP-11_architecture#General_re...
Auto increment makes a lot of sense in assembly language, for example when accessing memory so I can assume that C followed assembly
I've also seen n++ in pedagogical Analysis for the successor function.
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Relatedly, the PDP-11 addresses modes are post-increment and pre-decrement in order to allow programming the PDP-11 as if it were a stack machine, with a register pointing at the ‘top’ of the stack (i.e. lowest address). (Reference: Computer Engineering: A DEC View of Hardware Systems Design)