Translation:He hates beef because he is a vegetarian.
Apparently you like to rant on. None of my friends was alive in 1842. Several of them were educated through Irish to post graduate level. Foras na Gaeilge doesn't support one term over the other. It is not a dialect choice until recently the only choice was feoilséantóir'. Arguing that a term that is clearly based on a wacky version of English is more "modern" and should replace the existing word; in spite of the native speakers preference is ridiculous.
The correctnes of translations aside. The sentence is making a statment that is factualy incorrect. As a vegetarian, i still liked beef, I just preferred not to eat it. Rearranging the sentence to - He is a vegetarian because he hates beef (and inferring other meats) would at least make some sense
It is no more or less factual than any other statement on Duolingo. You aren't the only vegetarian in the world, and some vegetarians do hate beef, and there only has to be one beef-hating vegetarian who ascribes their hatred of beef to their vegetarianism for this statement to be true.
As feoilséantóir is a word that long predates the word "vegetarian", it's not accurate to say that feoilséantóir is the Irish for "vegetarian".
Many vegetarians who consider vegetarianism as a positive choice, rather than a penitential avoidance of meat prefer the positive term veigeatóir rather than the negative term feoilséantóir.