The linked article at the end says: "First, using Hallucination Check together with GPTZero’s AI Detector allows users to check for AI-generated text and suspicious citations at the same time, and even use one result to verify the other. Second, Hallucination Check greatly reduces the time and labor necessary to verify a document’s sources by identifying flawed citations for a human to review."

On their site (https://gptzero.me/sources) it also says "GPTZero's Hallucination Detector automatically detects hallucinated sources and poorly supported claims in essays. Verify academic integrity with the most accurate hallucination detection tool for educators", so it does more than just identify invalid citations. Seems to do exactly what you're talking about.