Those methods have improved too of course over the last few decades, but yes even back then it was quite accurate. It’s not terribly complicated. You take the image, see where it’s moving and at what speed and just forecast that into the future.
Weather reporting has gotten notably much more accurate during my lifetime.
You are making this sound way too simplistic - and sort of insulting to scientists who study weather patterns, it is not nearly as "draw a straight line and be right 100% of the time" as you're making it out to be, especially over larger timescales.
