The indefinite article is under-, the definite article is overused in Hungarian compared to English.

I think it does, but to me this feels more like "rich people eat in restaurants" (as opposed to, for instance, poor people eat at home), which, I think, is a more proper English sentence than the one proposed as translation here. The focus is on "rich people". I would translate the original Hungarian sentence (that is, with that word order) as "in (the) restaurants rich people are eating/eat" or even "in (the) restaurants there are rich people eating". The focus is on "in (the) restaurants". By the way, it's "éttermekben".