That's an interesting interpretation. I assume you mean as decendents of Abraham, Isaac and Jacob? I had never considered Catholics or christianity an ethnic group. Because to me it implied shared genetic features.
I completely agree with your comment, and not only that but also find your view on the quote interesting as well.
A lot of Jews would say that any Jew who becomes a Christian is no longer Jewish (by ethnicity or religion) even if said "Jewish Christian" still abides by and observes many Jewish customs and holidays.
I would argue that there are also many atheist Catholics (e.g. in Italy) who would disagree that they were not also part of an ethnorelegious group.
The majority of Jews today (in the ballpark of 75%) are Ashkenazi Jews which is a sort of 'race' in that they share a very recent common ancestor -- which is also why so many Jews share common physical features. All of this is very different than other religions, even other Abrahamic religions. So it is not inconsistent that state that Jews are an ethnic group, but other religious groups are not.
> If you want to convert it's a big deal with study, testing, and culminating with a literal certificate of conversion - that can be revoked.
https://judaism.stackexchange.com/questions/50602/can-conver...
Only in narrow areas, such as modern State of Israel (which is a blip in Jewish history, and not the world government of Jews), which very special circumstances.
It's complicated.
Also, you can be a non-Jewish woman, marry a Jewish man, and have Jewish children, who an Orthodox Jew might call non-Jewish children, but those children and their descendants and neighbors may never think are "not Jewish"
For instance, in the past interfaith marriage was not only frowned upon but something that would likely result in individuals facing exile from the Jewish community. This, for instance, is why most Jews today share so many distinct physical and other characteristics. In searching for some interesting numbers I came upon this article [1] which you might find interesting - it discusses the ongoing "reformation" of Judaism. The reason I put reformation in quotes is because I think it's "reforming" in the way that most religions are today - it's fading. And I think a century it will probably make no more sense to refer to Jews as an ethnic group than it would to refer to Irish Americans as an ethic group today. However, as of today Jews most certainly remain an ethic group by most all facets of the word.
[1] - https://www.nytimes.com/2013/10/01/us/poll-shows-major-shift...