Although the cited Gallup report doesn’t explicitly distinguish on immigrant status or ethnicity, it does say that “[a]lmost all segments of the U.S. population have similar percentages of vegetarians” while noting a larger difference in marital status.
As a brief example with easyish math, 15M out of 300M = 5%; 15M out of 330M (adding 30M extra meat eaters) only drops it to ~4.5%. Addition of 30M non-v*gan immigrants would mask an 1,500,000 increase in the number of non-immigrant vegetarians (15M/300M = 5% = 16.5M/330M). Without the 30M immigrants, the vegetarian population would have risen from 5% to 16.5M/300M = 5.5%. Given that the assumption that no immigrants are vegetarian is unrealistic, this shows that adding a good number of meat-eaters to the denominator doesn’t move the percentages much at all.
Although the cited Gallup report doesn’t explicitly distinguish on immigrant status or ethnicity, it does say that “[a]lmost all segments of the U.S. population have similar percentages of vegetarians” while noting a larger difference in marital status.
Even if one assumes that almost no immigrants are vegetarian, the rate of immigration isn’t so high as to really move a low percentage very much. As of 2018, there were ~45M people in the US who were born in another country. [https://www.pewresearch.org/short-reads/2020/08/20/key-findings-about-u-s-immigrants/]
As a brief example with easyish math, 15M out of 300M = 5%; 15M out of 330M (adding 30M extra meat eaters) only drops it to ~4.5%. Addition of 30M non-v*gan immigrants would mask an 1,500,000 increase in the number of non-immigrant vegetarians (15M/300M = 5% = 16.5M/330M). Without the 30M immigrants, the vegetarian population would have risen from 5% to 16.5M/300M = 5.5%. Given that the assumption that no immigrants are vegetarian is unrealistic, this shows that adding a good number of meat-eaters to the denominator doesn’t move the percentages much at all.