IMO it's not meant as a trick question.
I'd figured the reason you didn't respond was b/c it looked like a petty logical argument, but its implication is you can disallow immigration but you do need some rules to determine who gets to be in the country.
@
xerxe may have worded it at an extreme lol but I think he's not trying to imply that if you don't acknowledge the minor genetic differences within nations that you might as well go for open borders, mass immigration and multiculturalism. He's actually suggesting that implementing what you're saying won't be so simple, and might lead to an unfair result.
FYI I'm not trying to defend xerxe so much as clear up what looks like a misunderstanding. Will really try to stay out of other ppl's arguments in the future.
I'm putting together my own thoughts on the topic and will get them soon. >.<