Is it harder to transition from Industry Research to Prof than PostDoc to Prof? Assuming someone just received their PhD (in say, Computer Science) and chooses to step into Industry RnD rather than taking up a post doc, a year or two later, he/she realizes that Academia is where he/she truly wishes to be.

Is it tougher to transition from an Industry RnD job to a professorship as compared to a parallel universe in which he would have chosen a PostDoc instead?

Can an Industry RnD job double up as PostDoc?

(Assuming that one keeps publishing and works on important problems)
The "assuming one keeps publishing and works on important problems" bit is the hard part.  If you are working at a serious industrial research lab doing academic-style research, then moving to academia is doable.  If you are in a more typical R&amp;D job but doing cutting-edge work and actively publishing, then it's still possible (but you may face more resistance, depending on your subfield).  On the other hand, the vast majority of industrial R&amp;D positions in CS do not put one in a good position to apply for faculty jobs.

It's easy to be overly optimistic about this.  I've seen a number of students take industrial jobs and declare "I'm going to keep my options open by continuing to publish, even though it's not a job requirement."  This rarely works out.  It's far from impossible, but maintaining an active research program when it's not your primary job is difficult.