Did they realize/understand that a great portion of knowledge had been lost, possibly forever? Or were there still copies of most of the scrolls that were destroyed still extant in ancient times?
The burning of the royal archive in 48/47 BCE was a moderately newsworthy event as far as ancient writers were concerned, in that it made a memorable anecdote; and people who lived in Alexandria would certainly have lamented the destruction of a large chunk of their city.
But the second option you present is the correct one. There were numerous research libraries around the ancient Mediterranean. The comparison I habitually make with these questions is to suggest imagining that the Library of Congress were to burn down tomorrow. A certain amount of antiquarian material would be lost (first editions, manuscripts written by interesting historical individuals, etc.), but scarcely any actual information would be lost for good. Alexandria is basically the same thing writ small. Nothing of any real significance was lost as a direct result of the destruction of any one library.