Ok so this may be a stretch BUT…
I noticed when Joyce is talking to Will outside the barn about Robin, she says…
“There’s one thing I know for sure, she’s not your mom. She hasn’t seen what I’ve seen,”
Almost no one knows someone more than their own mother, and Joyce sees Wills most truthful self, who he really is and what is best for him!!! Joyce is saying that Robin might not know what is really best for Will.
Robin doesn’t see the truth of byler!
The script says “Mike doesn’t clock it. Robin does.”, which further shows that this may be what Robin is seeing in this situation. Not necessarily  that it’s her POV, but that she is taking in what she’s seeing.
What Robin sees in this shot of byler may not be the full truth, because “She hasn’t seen what I’ve seen,” OR the full truth of byler.
She is only clocking this little bit, leading her to give Will advice on what SHE has seen, not what the truth may be. She think’s what’s best for Will is to move past Mike and just focus on himself, but she doesn’t know what’s best for Will. She doesn’t know the truth of byler!
This also further supports that her monologue is NOT saying that “Mike is Wills Tammy”, because that is what SHE thinks based off of was SHE saw, not what is actually true. We know that Mike is NOT Wills Tammy because we know the truth and have seen more than just that little bit of byler…
We know the truth of byler and what is really best for Will.