"When Malay or Indian say the same thing"? What, where, when. I don't remember it on this forum. Care to post a link?
There are people (look in the mirror) who are mistaken (making such a simple mistake, like a 4 year old kid), but claim to be so "expert", and refuse to be corrected (unlike a 4 year old kid).
The people here trying to point out the mistake is not being defensive. They're trying to help you not make a fool of yourself next time. They are not language experts, and are not claiming to be. But the mistake being made here is the silly kind made by very young kids (or people who does not really know the language, but is overconfident and thinks too much of their understanding).
Now, just consider what happens if you're really mistaken, and that phrase really isn't a racial slur. As soon as you walk out of earshot, the entire staff in that shop will be laughing at you. 'So "action", thinks he understand Cantonese'. When Andy is at home having dinner, he will tell his family about this unreasonable customer, forcing him to appologize so many times, on his day off, for some imagined wrong. They will all join in cursing you.
Or you confront another shopkeeper, and he really let you have it (because, well, you deserve it) ....
Think of the embarassment when you repeat the same mistake when you visit a shop with a friend/colleague. After you leave the shop, they point it out to you, only for you to say "actually about 20 different people on this web forum tried to explain to me, but I didn't believe them".
Don't get me wrong. There are lots of racist Chinese people in KL and the rest of Malaysia. And they do call members of other races all sorts of epithets. But THIS isn't it. You're barking up the wrong tree. Come back here when you have a real example. Generally, this would be hard to get, because most people speaking the various Chinese dialects knows that people of other races who do not speak the language, nevertheless may have memorized the cuss words. So they wait until you're out of earshot before calling you that.
BTW: I think if your command of the language is good, when someone actually refer to you using a racist word thinkin you don't understand, the way is not to phone up someone else. You shout directly at the offender "hey, what did you just call me?" But you do it in Cantonese (or whatever language he was using). Boy, would he be shocked and surprised!

Wow so impressed with your explanation..
Coming frm a background that uses many local languages, sometime, we Malaysians can get confused either, as I also tot TS was slammed with a racist slur, until I put on my Cantonese hat did I et that the first word meant something else which was not racial.
My mother tougue, Thai has also many words which has different meaning when pronounced differently, same like Japanese, Korean, Vietnam, etc.