It all depends on how he watches them. Does he stare at them intensly?
If you would let him in with them, do you think he would chase them?
If he is relaxed while he watches, I don't see a problem. But if his bodylanguage is tense, you know he is thinking of chasing them.
Selfcontrole exercises and recall in front of these distractions might help a little
My two do the same thing, but I know they have the chase in mind when they do.
Especially Jinx... She once got away from me and went into a meadow full speed, to round up a flock of about 20-25 sheep. Being deaf to my recall
Very neatly she rounded them up and then stood there, not knowing what to do next. And then she came back to me.
So now I know she knows how to herd
She's not getting off lead when there is lifestock around