I agree with you completely, except for the conclusion from your prior post. Just as we get 9x9=81 from memorized multiplication tables, we do the same regarding 10x10. So, instead of using the numbers given and then applying the multiplication table, we change the numbers and THEN apply the multiplication table. Neither is really estimating, it seems to me.
That's true but, unlike 9x9, there is a way to actually multiply 10x10 without having memorized it. That was the point of my questions on the steps to do it and how you'd teach someone else to do it.
So, instead of using the numbers given and then applying the multiplication table, we change the numbers and THEN apply the multiplication table. Neither is really estimating, it seems to me.
I think that it is because you're changing the numbers to convenient numbers that are "close enough" to the original ones for your purposes. Doing that lets you easily generate an answer that's "close enough" to the real solution for whatever purpose you need. IMHO, that's what estimating is all about.
Sometimes, you need the exact answer and the estimate tells you the your final answer is reasonable. In other cases, the estimate is sufficient to meet your needs and it's a waste of time to calculate the exact answer.