maybe because Solitude is about the writing process. since the part about Pinnochio is a metaphor for the writer's solitude and creative process ("it is in this darkness... that the essentia creative act of the book takes place" )he uses an inkwell instead of a squid. Just as Pinnochio is dipped into the darkness (and creates himself), so Collodi dips his pen into the inwell and creates his work (which is also a way of creating himself).