First off:   Flagging link only answers is the correct thing to do.  
You may also edit the answer if you're so inclined. 

Second opinions can differ but in this specific [Q & A](https://serverfault.com/a/182605/37681) I would most likely have declined the flag too, because :

 - The answer is 5 years old

Not every old Q & A is worth judging to current standards. 

 - The answer is Accepted by the OP

So despite being nearly a link only answer the OP found it useful. 

Additionally such an answer is actually the best in this specific case, it is so because that is the standaard and reproducing (sections of) the standard would not really improve the answer as much as simply pointing there.

 - The answer is upvoted a number of times

and at the time apparently a number of other community members found it a useful answer too.

And last: AFAIK there is no "black mark" if your flags get declined.