People made missiles and precise engineering like jet aircraft before we had computers, humans can do all of those things reliably just by spending more time thinking about it, inventing better strategies and using more paper.
Our brains weren't made to do such computations, but a general intelligence can solve the problem anyway by using what it has in a smart way.
I'd wager that 95% of humans wouldn't be able to do 10x10 multiplication without errors, even if we paid them $100 to get it right. There's a reason we had to invent lots of machines to help us.
It would be an interesting social studies paper to try and recreate some "LLMs can't think" papers with humans.
The reason was efficiency, not that we couldn't do it. If a machine can do it then we don't need expensive humans to do it, so human time can be used more effectively.
But as long as AI cannot do that they cannot replace humans, and we are very far from that. Currently AI cannot even replace individual humans in most white collar jobs, and replacing entire team is way harder than replacing an individual, and then even harder is replacing workers in an entire field meaning the AI has to make research and advances on its own etc.
So like, we are still very far from AI completely being able to replace human thinking and thus be called AGI.
Or in other words, AI has to replace those giants to be able to replace humanity, since those giants are humans.
With enough effort and time we can arrive at a perfect solution to those problems without a computer.
This is not a hypothetical, it was like that for at least hundreds of years.
But then you're not measuring the ability to perform the calculations, but the ability to invent the methods that make the calculation possible.