That's a pretty strong statement, but even if we take it as an axiom, I think that it isn't migration that concentrates it -- people migrate from everywhere to everywhere all the time.
I think it has more to do with having gone through some hardship, and (selection bias) having done so successfully, makes people better at taking risks.
There's a statistic that says that a huge number (ten percent or so) of immigrants immigrate again to a 3rd country.
Someone who never immigrated would think "all that trouble, again?". Someone who [successfully] has thinks "Well, that wasn't so hard, we can try that again".
I think you missed the point- he meant that the fact that you were able to immigrate probably means that you're not the average person. He's right- immigration is much easier if you're rich, smart or both.
Also, I'm offended by your last sentence. You've obviously not immigrated to america recently. I have, and it wasn't easy, nor was it cheap.
As a matter of fact, I did. It's not easy, it's far from cheap, but having gone through all of that, another move -- at least to an English speaking country -- is comparatively trivial to what I've thought about it before.
And if you take offence at random remarks by strangers on the internet, you should have a long talk with yourself.
I meant that it appears to be random (or even anti-intuitively correlated) if you look at one factor only: I believe you need both genetics and environment.
The anti-intuitive thing is what others have said: migrants are bolder (entrepeneurish) or they have some requested talent.
As an ex-immigrant (returned to India now to start something), I believe this is true. US immigration policy weeds out the chaff. The immigrant populations from countries like India tend to also be homogeneously upper class/caste.
Can be parental, culture, education, standard of living, etc.