"László is a Hungarian writer."
Translation:László magyar író.
Yes, it is implied. In the third person, when the statement is a noun-like clause (sorry, don't know the exact term) or an adjective, the verb "van" is omitted. And this is not optional. When it is an adverb, a location, etc. then "van" is necessary.
László író - László is a writer.
László éhes - László is hungry.
László otthon van - László is at home.
László jól van - László is well.
It may be a better translation but I wouldn't say this is because of the "is a". Sometimes it's actually more natural not to include "a" in the Hungarian version and I feel there is a nuanced difference here as well - although in my opinion, that difference makes the "egy" version sound a bit better.
Anyways, you might come across people here on DL who want to simplify stuff by saying "egy is optional" or something like this but I'd ask you not to listen to them, in order to avoid further complications. Sentences don't sound the same with or without "egy" in most cases, to the extent one of the versions may sound less common/even unnatural. It's more like there is a legitimate third option besides using "a/az" or "egy" which happens to be more similar to "egy" and therefore it mostly translates to using "a/an" in English.