I routinely ride several short, steep hills that are 18-20% in places. I am in my granniest of granny, but I am not standing. So, I don't think you can say most recreational cyclists can't do this. It's all what you are used to. My driveway has a spot that is 15% and my street has some 10-12% grades. Believe me, I am not a racer, etc., just someone who lives around hills.
Now, while I can handle this, I don't like long climbs, i.e. mountains that go on forever, even if they are less steep. And the only time I got off my bike and walked was a sustained 18-22% grade for about a mile. I rode about 1/4 of it.
I have also found that my DH's Garmin estimates the grade as about the same as any other method we have used.