Hop addition question

I am going to be brewing up Heritics Shallow Grave Porter clone this weekend... It has a late aroma hop addition for the last 15 minutes of the boil. I was curious as to why it recommends the addition at the last 15 minutes, rather then flameout? I was under the impression that you get better hop aroma at flameout, even beyond flameout, say cooling down the wort to 180 and then steeping the hops..

Should I just be following the recipe with the hop addition?

I'd follow the recipe. You get more flavor and aroma with the late additions but you also get less IBU. And I'm pretty sure hop isomerization stops at 190f

A 15 minute addition will give you flavor and little to no aroma. A flame out or hop-steep addition will give you mostly aroma with little to no flavor.