There is a big difference in value judgement between "Close Enough" and "Difference Not Much". "Close Enough" is a value judgement on the customer's behalf, meaning that a factory representative is presuming to know what the exact customer (and the customer's customer, etc) requirements are, maybe even suggesting that th factory knows better than to follow the written specifications!
"Difference Not Much" just means that it's hard to tell the difference, without any inherent value judgement on whether the product is acceptable or not. It's not a fundamentally offensive statement in the way that "Close Enough" would be.
I think that the correct and polite response to "Cha Bu Duo" would probably be "Cha Bu Duo, but the small amount of Cha that is there is very important to our customers, etc..."
I also think this article is a little out of date, some factories in China have definitely been churning out some pretty good stuff for awhile now...