It's not an exploit.

"Clever use of game mechanics" = What happened in FFXI with Ninja. They where not meant to tank.

Exploit = Making it work differently to intended.

Now let's check here.

Are those mobs part of the fate? Yes.
Are they intended to be killed? Yup.
Did anyone do anything to make them act in a way they shouldn't? nope.
Did a player make them not count towards completion? No. S-E coded them like that.
Does killing them break the fate in anyway? No, it's still completable and mobs work as they are intended.

Exploiting is bad, mm'kay?
Of course, if you go by the definition of exploit when it comes to gaming. If you go by what the word really means, then no, it's not bad.