Of course the flag doesn't define the person. What defines the persons comes to a subjective field which I don't wanna get in. Nationality doesn't make a person unique, in my opinion, there are a lot of aspects that could make a person unique, not just the nationality. And if u're saying that nationality is what makes a flag unique, I think it's a tautology.
Since nationalities are a way to cohesionate populations, grouping them into a territory and a culture in common, differentiating themselves from other ones, one of the symbols which associate people with their nationality is the flag. Here, in HLTV, the first method people have to differentiate their nationality from others, is by putting their own flag, It makes even more sense since there is only one lang available.
Now, if I'm saying I'm argentine by selecting the argentinian flag, I could be lying, of course, and that's why he created this topic, asking us to give some more proof about our nationality instead of just showing our flag. It implies some cultural manifestation which corresponds to the own nationality.