Here's a census of immigrants roughly around the time Tesla was working in the U.S. (1910's), showing the relative percentages from each region (a point of comparison which is not even accounting for the ones who came from the above regions before this time): https://www2.census.gov/library/publications/decennial/1910/...
It's good to read the facts of history rather than hear trite historical narratives. In this case, it seems inbound immigrants from the Balkans made up just 1.6% in the 1910's.
I'm pretty sure the intention of saying a last name is "all-American" is to comment on the relative statistical frequency of it in a population at any given time, since that is a name people in another country (like China) would associate and know because of their likelihood of seeing it. But again, a lot of people discuss historical narratives rather than historical facts, so the above poster is not uncommon in continuing that tradition of pushing a trendy idea rather than discussing facts and information.