If they had been factual I the credit I'd agree. When it's actually a fork, why not just say so. "This project is a fork (or based off) Spegel. Thanks to the authors etc" Maybe with a rationale why they forked it. You know, just common decency...
Maybe not a fork, but the author writes "It looks as if large parts of the project were copied directly from Spegel without any mention of the original source".
