That's a bizarre "rule," and I've never heard of it. I sometimes use the format others have mentioned, and that is to write out the number (if less than 10), then place the numeral in parens after it. E.g., "It takes two (2) to tango." I only do that when there's a possibility for confusion, I'm writing something technically oriented, or I want to be as precise as possible.
If the client wants that, the client should get it, but I personally think it's weird! And, I agree than numbers in parens mean, in accounting speak, negative values. It's not just in Excel. Back in the stone age, when ledgers were done by hand, that's how negative values were indicated.
I learn something new every day, but not everyday!
I've learned to never trust spell-check or my phone's auto-fill feature.