Calling prompt injection "not malware" because LLM behavior is unpredictable is like saying a phishing email is not an attack because humans are unpredictable.
Even if maybe the mechanism of "injecting a prompt" could be beneficial in some use-cases, e.g. to instruct an LLM positively, this is case is clearly malicious by intent. The author even tried to hide it by obfuscation.
It's just an insane take by that libraries author. Even someone "on their side", that may even hate AI/LLMs more than him, would probably drop that library in a heartbeat, as the authors judgement clearly can't be trusted.