Of course you’re correct in one sense. However, science is an old word and like many others in the English language it has multiple senses. In the context I used it in, it means a systematic body of knowledge. I’m sure you’d agree mathematics meets that criterion. Other examples include ethics and aesthetics.
Dijkstra and Scholten would beg to differ. Mathematics is the science of describing patterns, and mutable state falls well within its purview.