Closing it as a duplicate presumes a specific correct answer.
The argument is that it's redundant to ask question 2 (is the echo a creature?) because the answer to question 1 (does it provoke opportunity attacks?) already entails that. This envisions the answer to question 1 as a syllogism:
A. Only a creature can provoke opportunity attacks.
B. An echo is not a creature.
Therefore, C. An echo does not provoke opportunity attacks.
If that answer properly establishes its premises, it will contain an answer to question 2.
But this is not the only possible answer. In fact there are currently three answers to question 1:
- the accepted answer ("no"), which relies on that syllogism;
- an overwhelming upvoted answer ("yes") that directly contradicts the accepted answer;
- a moderately upvoted answer ("I don't know") saying the language is ambiguous.
Now, assuming the validity of premise A, the "yes" answer must also imply an answer to question 2--that the echo is a creature. However, the argument actually made by the "yes" answer is that premise A is unreliable (and it goes on to answer question 1 by considering other factors).
Closing question 2 as a duplicate therefore amounts to affirming premise A and rejecting the "yes" answer to question 1, and probably the "I don't know" answer as well.
This is not what closing questions is for. It's a janitorial tool for keeping the space clean and maintaining the quality of questions, not a device for declaring victory in arguments.