I don’t think anybody said that Arabic has suffered a complete standstill, and it has doubtlessly evolved significantly.

But if you compare it with basically any other major language, it’s clearly much, much more conservative. If you are a native English speaker, understanding English from 1,000 years ago is like learning a completely different language. If you are a native speaker of Italian, you cannot understand a text in Latin without significant training. This is true for all European languages other than Icelandic.

Chinese is pretty similar, even though the written language is slightly more stable.

So in comparison, Arabic is incredibly conservative.