Hmm...this is interesting, cuz for me as a native English speaker, the first thought that came into my mind about the meaning of this sentence is the 'their' is referring to a company which makes beer, and in English, one could say, "Their beer tastes good," and it would be understood to mean that the beer the company made or that the bar sold to you is delicious. Alternatively, this sentence could mean, the beer that they are drinking is delicious, but that sounds (to me) like there is ONE beer drink being shared by two or more people ("their beer"). Could one say "ilia[j] bieroj" to mean "their beers" (i.e. each individual person's beer)? Or is 'beer' merely treated as a non-countable noun in Esperanto, like 'water'?
Well… I don't know how fake Latin, Greek, German and Slavic is, but that is where many of the roots of Esperanto can be traced. For example; Bon~ (the first syllable in Bongusta) is pretty much directly from Latin, though there are other European languages which use that radical also. French uses Bon, Spanish uses Bueno, and Portuguese uses Bom. Even Romanian uses Bun while on Corsica and Sardinia (2 Mediterranean islands) people say Bonu.
Meanwhile gust~ comes to us, through the Latin Gustus, from the HindoEurpean root Geus (= satisfy, eat). It has cognates in (I wish that I had the correct fonts here) Sanskrit Ĝuŝ and Greek Geso, (both meaning ŝati) and the English Gusto (= vervo, entuziasmo).
You know what, I'm going to assume that "fake" in your post was a typo.