“As Robert Frost said, ‘Good fences make good neighbors’.”

This is just true enough that it trips up even people who ought to know better. The line is from the poem “Mending Wall”, about Frost and a New England neighbor repairing the stone fence between their property, once Spring has arrived. Frost is not opposed to the idea of a wall, but he is puzzled why the neighbor insists on rebuilding a portion of it between two groves of trees, none of which are likely to stroll over to the other person’s side. The man replies, “Good fences make good neighbors.” Frost finds this rather foolish:

Before I built a wall I’d ask to know
What I was walling in or walling out,
And to whom I was like to give offence.
Something there is that doesn’t love a wall,
That wants it down.

Watching the neighbor, he gets the sense that the man doesn’t grasp the POINT of a boundary, and is content simply to parrot the expression, without comprehending or analyzing it:

He moves in darkness as it seems to me ~
Not of woods only and the shade of trees.
He will not go behind his father’s saying,
And he likes having thought of it so well
He says again, “Good fences make good neighbors.”

So yes, Frost does “say” it, but he puts it in the mouth of a self-satisfied twit for whom he has a certain amount of contempt. “Saying it” is not the same as ENDORSING the idea.