We know CW mutants can be articulate (Leo and Fawkes), and some of the idle pvssyr you can listen to gets pretty deep. Low verbal intelligence doesn't constitute low tactical or other intelligence. By reasoning, we can assume Super Mutants can sneak by and ambush patrols since they actively re-populate areas. Their defense of the front of Vault 87 shows they know that area is lethal to humans, and their defense of the "secret entrance" shows they know it's a vulnerability. If I recall, they even use traps! Their repopulation also lets us know they are kidnapping people all the time and know how to make them Super Mutants.

"I'll wear your skull around my neck!" is actually a far more intelligent statement than you might think. It shows planning, foresight, and imagination.

True but when we here most of them speak long sentences it is uneven and stilted as if their struggling to put it together, this one while still having the guttural tone doesn't have those same issues.

I suspect the Institute started making FEV for some research purposes and surviving Vault 87 mutants managed to secure it. Because this source doesn't require dragging victims through radiation fields they come out a bit smarter. Even if they originated from CW they must have secured a source of FEV in the Boston area. They may also be a strain that originated entirely in the Boston area.

Well again he could be like Uncle Leo or Fawkes, furthermore there could have been other "smart" super mutants who exiled themselves.