Derivative is a pretty straight forward word, meaning one thing directly came from another. In this case it means that rather than designing this myself, I just copied Syndicate.
I'm not so much offended myself, because I get these comments all the time and get used to it. You will make some people very angry though, because it does question their creativity. When an artist spends time to be creative and design something it's disappointing when someone then tells them their design isn't creative. In my experience when a comparison is made it's almost always incorrect, as in the artist did not use that inspiration. So it's just some simple advice: When you see a similarity in something and feel like pointing it out, it's probably best to not say it at all because it's probably not related and it's not considered a complement.
Not mad, just trying to educate