Related (thots), so perhaps useful.
First , the writer is spot on saying that the fingers don't have to be parallel to the keyboard, because initially I was "lining up" my hand as if fingers were cutlery in a slotted drawer. Your hand can adopt all kinds of angles vis-a-vis the keyboard. I felt like an idiot when this was pointed out to me because it seems so obvious.
Another thing is distance to the piano. If you are too close you'll have to angle your elbows out to make room for your forearms and you'll get that kink at the wrist. Experiment with distance and what that does.
You also have instances where for example your LH will be way over in the low registers, the RH way over in the high registers - you'll be leaning forward a bit, and your arms will be angled. Or what if both hands are high up in the treble clef, and you're sweeping along toward to middle, say in an arpeggio. In other words, you're playing an instrument that is bigger and definitely wider than you are, so you're "reaching over" to different spots.
When I played originally as a child, the only book(s) I had kept to the area of middle C and I never developed that spatial awareness. Your hands at the wrists are attached to the rest of you. That too had not been in my awareness.