Discussion is an exchange of knowledge; an argument an exchange of ignorance.

What does "an argument an exchange" mean?

See a translation

It means "an argument is an exchange"

The word "is" is omitted. This is somewhat common when the second sentence/clause follows the same structure as the first sentence/clause. It's an optional style choice and it does not change the meaning, only the feeling. In my opinion, it makes it sound a little more formal, or a little more authoritative, or a little more poetic (depends on the situation).

Examples:

"Sadness is a rain storm; a friend an umbrella." (= A friend is an umbrella)

"Using a complicated password is a wise choice, never using the same password twice a wiser one." (= Never using the same password twice is a wiser choice)