The speaker is addressing the world, and calling it foolish for believing something: that the wind is dead. The speaker doesn't buy it. The wind is described as an "old and most ingenious acquaintance," someone known well and trusted to be clever, the sort of friend who might fake a disappearance and then turn up again. That affectionate, slightly teasing tone is what makes the poem feel personal rather than merely descriptive.
The second half offers the evidence for the world's claim. Snow is falling, and it seems to be "talking peace," the way evening light does. Everything has gone soft and quiet. But the word "talking" hints at something. Peace is being spoken, not simply felt, so the calm may be a performance. The title, "Lull," points the same way, since a lull is a pause, not an ending.
The mood is hushed and still, with a trace of wariness underneath. The speaker enjoys the quiet but doesn't trust it, and seems to be waiting for the wind to return.