It's actually quite plausible. Most writers on shows usually have an outline of how the ending to their series is going to look years in advance.
Of course things can always gets changed or revised in the process that drifts away from the original plan (and they usually do).
If Ash was always meant to have an Incineroar then I can see why most people would that early concept art was some kind of foreshadowing.
As for Popplio, there's always the possibility that it wasn't originally intended to evolve. But Primarina's massive Popularity in Japan might've caused a change in plans
However, Mallow kinda implies it was Game art instead of anime art, since it makes no sense she is up there since she didn't get any starter.