The speaker and a companion have been left alone, ignored by the people who once judged them and turned out to be wrong. "They" have written the pair off and moved on, leaving them to follow their own path. You might expect bitterness. Instead, the two of them sit down in a roadside nook and, with a playful, almost angelic grin, try to feel abandoned. They can't quite manage it.
That's the joke at the heart of the poem. The words "mischievous, vagrant, seraphic" make them sound like happy truants, not wounded outcasts. Being neglected has set them free, and they treat the loneliness others assume they feel as a game, a role they test out and find doesn't fit. The mood is light, wry, and quietly defiant. The tight rhyme, with the same sound repeating through five short lines, adds to the feeling of a private, amused secret.