Yes, it is. Zen as such is a religion and philosophy from Japan; it is intrinsically Japanese, by definition, just as Judaism is by definition intrinsically of the Hebrew ethnicity.
I am aware of the use the term has taken on in popular culture, divorced from its original context -- that's precisely what I'm complaining about.
This trend is nothing more than ongoing cultural appropriation of exotic cachet and a watered-down stereotyped design aestheic from a faraway foreign ethnicity, in order to seem cool and sophisticated to other westerners. Websites labeled Zenwhatever have nothing to do with actual zen. It's a philosophical blackface minstrel show.
That the stereotypes being promoted about an ethno-religious identity seem to be positive and flattering is irrelevant. Few people would call their accounting software "Jewbook" to trade on a supposedly positive and flattering image of Jews as prudent financial managers -- yet "Zenefits" is supposedly fine.
Indeed, 50 or 100 years ago, many people widely used the word "Jew" as an adjective in a positive and flattering sense related to "having business acumen" or "being a good negotiator," and when challenged offered very similar defenses to the one you just offered, about how "jew" had by then become a generic word for something good, divorced of its original context, and not a religious stereotype or insult in any sense.
In Japanese culture, incidentally, Zen is most typically associated with Japanese racial superiority ideology and far-right wing militarism. How ironic considering its cachet among young, cool progressive types in the west!