A device for what? I mean, I guess it could be interpreted as writing love letters by hand instead of lyrics, but given Brian was talking about the name because he wrote everything by hand, I don't see how you can't say its at least parcially about the lyrics. Self-servingness aside, it still isn't a great song. the 'every word handwritten' part gets rather repetitive. It isn't bad, just as this point I'd say its my least favorite gaslight song. Has a few lyrical bright spots, but in general just feels flat.
It's a metaphor for really believing in what he was writing. He's said in a bunch of interviews that the reason he did actually write everything by hand was so he'd agonize over each line, making it as personal as can be. These songs were "handwritten" in the sense that they are fully his, everything is out in the open in black and white. There are little to no literary references, borrowed lyrics, etc that he's used so frequently in the past. Saying "every word Handwritten" is just his way of conveying that idea. At least that's my interpretation, take from it what you will.