Well, here Frost seems to be spelling things out, making a declaration, making a statement, saying what the fact is, and seeming to celebrate the literal: "the fact is the sweetest dream that labor knows."

这儿弗罗斯特好像要详细地说明，来宣布一个声明，说着事实是什么，似乎要庆祝原有的意味：,事实是劳动者知道的最美丽的梦“

耶鲁公开课 - 现代诗歌课程节选