This so called ‘estimate’ didn’t even use the rules of rounding correctly.
The old fashion drill and kill method of learning ones ‘tables’ did more to help teach arithmetic (which this is....it is not ‘mathematics’ as such).
We wonder why people can’t do calculations when their quackulator dies and we now know why.
This is malfeasance of the highest order
My boys used to ask, “How long till we get there?” on trips. My answer was the number of remaining miles and my current speed.
They would say, “Nevermind”, to which I would reply, “Nope, you started it. Tell me how long.”
“But I don’t have a calculator.”
“Too bad. Do it in your head.”
One is now an architect, and the other is an engineer.
I agree with your basic idea, but suggest that arithmetic is one branch of mathematics, not something separate.