C'est nul vs Il est nul

C'est nul vs Il est nul

The quiz asked "It's lame", "_____ nul".

The answer that it wants is "C'est nul".

Why can't it be "Il est nul"?

Suppose the sentence was a response to the question "What do you think of that film?"

The answer is providing an opinion with an adjective which it is applying to a specific thing - "that film".

That sounds a lot like case 2b in the lesson:

"2. Cases expressing opinions or simple statements (adjectives) about prementioned things"

"b. il est/elle est  for statements and opinions related to specific things"