This usage is meant to mean 'so' to some extent and is often interchangable. I think it comes from 'There isn't so much to do that I need your help.'
'so' is used to express a certain extent or 'very.' 'that' is used when there is a defined extent or an understood amount.
I think it's best to tell them it means 'so' but a defined amount. Then I would just give them loads of examples.
I don't go there that often. (I don't go there as often as some people, as often as you, often enough to know the waiters, etc.)
It's going to cost $1,000. We need that much. (that = $1,000 worth)
There isn't that much to do. (so much that I have to do it right now, so much that I need your help, so much that ...)
I hope that helps.
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