Learn Dutch: Zulk werk is authentiek. - Such work is authentic.

The word werk is a singular neuter noun (a het-word). Categorical demonstratives like zulk only get an -e ending (zulke) when they come before de-words or plural nouns. For singular het-words, you must use the uninflected form zulk.

This lesson is about dergelijk(e), so why is this sentence using zulk?

Zulk and dergelijk both mean 'such' and are largely interchangeable. Dergelijk is slightly more formal, while zulk is more common in everyday speech. You could perfectly well say Dergelijk werk is authentiek here, which also uses the uninflected form because werk is a singular het-word.

Why don't we use zo'n (such a) here?

We use zo'n for singular, countable nouns (like zo'n taak for 'such a task'). In this sentence, werk is treated as an uncountable noun (work in general). For uncountable nouns and plurals, we use zulk(e) or dergelijk(e) instead of zo'n.

Does the adjective authentiek remain without an -e ending because werk is a het-word?