IOW, why handed vs ambidextrous, not so much why left-handed vs right-handed.
Did it even explain that? I'm ambidextrous, I have no handedness bias, so whichever I pick up to first learn something is the hand I use. So I'm a mix of left-handed and right-handed depending on the task. And yet I didn't really understand why that's odd because of my bipedalism?
Mildly informative Wikipedia article: https://en.wikipedia.org/wiki/Cross-dominance
I'm right-handed. I'm definitely sure of that. It's just that what hand I use depends on which hand I start with from the beginning. And that'll be the preferred hand, except for things where it's natural to switch from the very beginning.
But there are also some things I've learned to do with both hands much later, e.g. washing dishes - there was a reason for having to do that for a while, and now I just switch when I like it, or if the kitchen happens to be arranged in a way which makes one side more preferable.
(BTW, when building carpenters still used common hammers it was completely normal to use either hand, as access space may be limited and there's basically no choice sometimes.)
Ambidextrous means: "that the person has no marked preference for the use of the right or left hand".
Unlike what is normally assumed, it doesn't mean you've mastered both hands, it doesn't imply you can write equally well with the left or the right, but that you could have just as easily learned either. You had no marked preference when you picked up the activity initially.
Cross-dominance seem to imply there was a marked preference, but that it is the left or right hand differs per activity.
That said, I wonder if they're really the same. Often people say ambidextrous as you are equally good with both hands, which would always require practice on both side. But maybe cross-dominant people can equally learn?
Under that reading, cross-dominance could be what ambidexterity actually looks like in practice. If you genuinely have no preference, there's no reason you'd consistently land on the same hand across all activities, you'd be influenced by context, who taught you, which hand was free, etc.
Have you ever tried your other hand at activities? And are you surprisingly good with it, even if not as good? I tend to be better using my other hand at most things than say a fully right handed person, but never as good as the hand I've been using for that activity consistently.
may be high dexterity is expensive, brain-wise, i.e. may be the choice given average brain is either 2 hands with mild dexterity or a one of high dexterity at the expense of the other. With tools, etc. the latter choice seem to be preferable and was selected for (and the lucky ones get to have 2 of high dexterity) Bipedalism and brain expansion in this situation are indirectly connected to the handedness as they are enablers and drivers of tools use.
Neither of my parents were left handed, and yet, here I am; despite attempts otherwise I might add.
As for which hand is chosen as a baby, it might be most typical to mirror one's parents, but it doesn't always have to be like that. Some babies may just randomly start doing things with a hand different to the preference of their parents due to not having them or other adults in their sight at critical moments. As for parents trying and failing to stop their kids from using their left hand, they might've just caught it too late.
So, I guess what I'm saying is that both you and GP can be correct.
For example, throwing (or kicking) with your non-preferred side is not as simple as picking up and throwing a ball or simply kicking it. You have to adjust your position and stride to lead with the correct foot. I found learning right-handed pace bowling in cricket (for fun) especially challenging as you have to land your back foot in the right place as you bowl through the popping crease. A few steps and rolling the arm over to spin was easy, and I actually can get more spin on the ball with my right hand.
My theory is that the handedness came about through learning basic survival activities such as running and jumping, throwing spears or rocks, etc that require using a preferred or learned hand.