"They want to demolish this Pruitt–Igoe complex."
"What? These buildings? They are still quite new but badly maintained."
In this context this is a sentence. ;)
– Le akarják bontani a Pruitt–Igoe lakótelepet.
– Mi? Ezeket az épületeket? Szinte még újak, csak nincsenek karbantartva!
For more about the context see https://en.wikipedia.org/wiki/Pruitt–Igoe
I always wish if Duolingo had at least basic context hints. This is an ubiquitous problem. I tried quite some courses, and all of them suffered it.
See my earlier comment above -- the -t on the end of both ezeket and épületeket means that "these buildings" are in the accusative or object case. For a statement on its own like "Are these the buildings?", you'd need the nominative or subject case, with no final -t: "Ezek az épuletek?"
Basically, yes. This sentence is a bit odd without that context, but yes, it works well as the response to someone else. Something like, "[Person A]: I'm looking for certain buildings..." → "[Person B]: These buildings?" In this scenario, the "buildings" would be in the accusative, as the object of the unstated verb phrase "looking for". The Hungarian sentence is this kind of response.
Because ezek means "these" and not "those"; because the Hungarian sentence has az in it (so it could be, at best, "Are these the buildings?"); and because the words have the accusative marker -t on them, which they would not have if you are asking "Are these (the) buildings?".
That's right, it's not enough.
As for "why", I'm not sure that there's a good answer beyond "that's how Hungarian grammar works".
Why do we say "three girls" in English? Isn't "three girl" enough? The "three" already says that there are more than one -- you don't need to say "girlS" with an -s for "there are more than one" as well, do you?
Why is it il mio cane in Italian? Isn't mio cane enough? Like "my dog" in English -- if something is possessed, it's definite.
három lány is good Hungarian but "three girl" is not good English. Similarly, "these buildings" is good English but ezeket épületeket is not good Hungarian. The two languages' grammars simply don't work quite the same.