> Any child who read maths textbooks with enthusiasm for 3-4 hours a day for years could in theory at least get close to doing what he did

No, they couldn't. And neither could most adults, for that matter.

Innate ability is real.

The important factors seem to be intrinsic motivation and other good mental faculties like great memory for concepts and formulas, understanding.

It's hard to say whether the motivation came from the good skills (understanding, memory) e.g. "I'm good at this, I like it!", or that the good skills came from the motivation. I believe both are important though, and that they are intertwined.

No .. not really. Not even close. Just like even if I practiced music 8 hours a day I wouldn't be able to come up with the music Kurt Cobain has or Mozart. There are plenty of musicians who try really hard but lack the innate talent - at best they can learn to play other people's music but never can come up with good original music, at least not something other people want to hear.

As someone wrote here innate ability is a real thing

There is a massive body of research showing this is not true

I think it has to be both. You need some ability to understand and thus find happiness in the thing that you are reading which leads to the motivation.

  Any child who read maths textbooks with enthusiasm for 3-4 hours a day for years could in theory at least get close to doing what he did, but what kid had that level of motivation?

There are probably hundreds of people on this site who had the same enthusiasm for math and time dedication as Terence Tao, but lacked his extreme outlier fluid intelligence, processing speed, perfect memory, and even handwriting talent(!). Terence Tao mastered calculus at an age when most future-mathemician geniuses weren't yet strong readers of chapter books.

Another requirement is the emotional capacity at 8 years old to focus, feel confident, and feel safe.

I think that is the main obstacle to most people doing highly effective work and putting in long hours. You hear some call people who don't 'work hard' lazy, but my impression is that it's emotional capacity, and a lot of that comes from family.

I wonder if there is a correlation between prodigies and emotionally stable, healthy, present parents. It's hard to imagine children under a lot of stress - e.g., from abusive parents, highly unreliable parents (e.g., overwhelmed by addictions to drugs), emotionally unstable parents (e.g., narcissists), highly neglectful parents (e.g., who abandon their kids) ... - it's hard to imagine those kids doing what Tao did, regardless of their talent.

I take it you've never met another human before