Is this pure word porridge, or a nearly incomprehensibly hypotactic but nevertheless grammatical sentence?

You might want to read Carroll's Symbolic Logic. He enjoyed these sort of nonsensical syllogisms. The quotation from Alice, also seems to be poking fun at the British English penchant for double negatives being a sort of augmented positive, (e.g., "this is something I am not unfamiliar with").

Ceci n'est pas un seing.