In Act II.ii of King Lear, Kent (disguised) is taking Lear to Regan in hopes that she could lodge her father and his troops. Instead, Kent is placed in stocks because of what he says to Oswald. At the end of the scene, Kent discards his disguised prose speech as Caius and reverts back to poetry, resuming his true self, Earl of Kent, in this soliloquy. So, this statement is full of perspectivism: what looks mundane to some (a letter) looks like a miracle to those in misery (Lear, Kent).
Granted, Cordelia's letter is only a letter; it's not like it is a Deus Ex Machina (it will not save them from a hopeless situation), but it is a kind of miracle. After all, Cordelia is the hero of the play, a potential Christ-figure.
That someone like Cordelia knows and empathizes with their suffering gives Kent a kind of hope. So says Enotes:
Left alone, Kent is optimistic about his time in the stocks. He will catch up on some much-needed sleep and the remainder of the time he will spend whistling. Before he sleeps, he finds comfort in reading a letter from Cordelia.
Critic A.C. Bradley has this to say about Kent's statement:
This, says Kent, is just the situation where something like miraculous help may be looked for; and he finds the sign of it in the fact that a letter from Cordelia has just reached him; for his course since his banishment has been so obscured that it is only by the rarest good fortune (some-thing like a miracle) that Cordelia has got intelligence of it. We may suppose that this intelligence came from one of Albany's or Cornwall's servants, some of whom are, he says (III. i. 23), "to France the spies and speculations Intelligent of our state."