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...
Of course, because both groups view themselves as belonging to the same group of people, with shared customs, history, religion and tradition, that dates back to the Kingdom of Israel. What you don't seem to understand is that ethnicity is a social concept - the criteria isn't "how much two people have in common", but whether a shared identity exists between them. You should ask yourself what makes you refer to both Ashkenazim and Mizrahim as "Jews", and what makes them refer to themselves as such, even among people who don't believe in Judaism.
> Well, Italians and Spaniards traditionally share a religion (Roman Catholicism), speak related languages, and share some biological ancestry – does that make them one ethnic group?
If they share the same ethnic identity then of course.
You want to focus on subjective questions of identity, but it isn't clear to me that all Jews have the same subjective sense of identity. Does a secular Zionist in Tel Aviv have the same subjective sense of identity as a Satmar anti-Zionist in Kiryas Joel? Certainly their "subjective sense of identity" has completely different ideological foundations. They likely wouldn't agree on who is a Jew either, since their different ideological foundations would lead them to different positions on issues such as conversion standards and patrilineal descent.
This doesn't of course mean that their "subjective sense of identity" is identical in its entirety, but that's pretty much the case with every ethnicity.