certainly talented people will have lots of different options as to where to apply their talent
but lets not pretend that all the places to apply talent are inherently equal, or that that the value propositions of those places is merely a function of whether those places are perceived as welcoming
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though its a highly imperfect analogy to a county, Harvard and Stanford regularly tell 95% of hopeful student that they're not welcome to come and study there, seemingly with little effect on their ability to attract the talent that they do want
I think that does indicate that talent attraction doesn't neatly equal welcomeness
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also seems worth noting, if one of the US's large assets in attracting talented foreigner is its large market available for entrepreneurial ambitions, one of its main competitors in this space doesn't have any issue categorizing entrants in their society based on their likely usefulness for their hosts https://www.nytimes.com/2016/09/22/world/asia/china-work-per...
It has come to symbolise it, in a sense of "no more Mexicans." But op specifically meant it literally, as in: be strict about visa enforcements. If you have a wall, but a very liberal immigration policy, that's not racist.
Again: you're right in other situations, but in this specific case, he was playing with the words.
regardless, the problem is if you are perceived as not wanting immigrants, the most skilled ones will go elsewhere, because they can.