Over-planning is definitely a problem — it can be a really sophisticated way to hide from doing the work of building something and showing it to people — and calling a plan a guess can definitely help pull the stakes down. At the same time, under-planning and wild-ass-guessing can be just as problematic.
All plans are based on some assumptions, and those assumptions are often where the guesswork is happening. That said, a good planning and evaluation process helps illuminate what those assumptions are, and working the plan confirms or invalidates those assumptions.
But saying that plans are just guesses is like saying science is just what some folks believe. It’s true, of course, but how they came to those beliefs matters.