People fall gradually more as inclinaison rises. There is no arbitrary cliff as 20° is crossed. Plus, minor issues but not insignificant issues like slippage probably started at very low level.
You have artificially transformed a continuous issue into a threshold which is intellectually shoddy.
That absolute is actually intellectually shoddy to me when parent is clearly using "could" and the slope to indicate it's around 20°, not at 20°. It doesn't really matter what the error margin is if it's low enough.
Let's try another analogy: of course the patient looks fine, but he's cancer terminal and in his last weeks. Tests are showing his internals are fucked up beyond certain thresholds. Yeah, they are smiling and talking but it's over, they are dead in two weeks top.
Omae wa mou.