Neanderthals probably did have some form of language since appear to also have had a gene that is crucial to language in humans. And they buried their dead. So why would we presume they didn't make some sort of marks intentionally, to convey meaning or just to decorate their favorite rock?
To me the more interesting question is: did Neanderthals value art?
Could argue all day about what is and isn't art and if they created artifacts that fit the definition, but what I'd really like to know is "did they appreciate things purely for aesthetics and cultural relevance, and not utility?"
There was an episode of In Our Time about cave art that features a discussion about the art not being solely for utility. Unfortunately I cannot find a transcript and I can't remember when the discussion came up. Still, if you are interested you might want to listen or look at some of the further reading links.
https://www.bbc.co.uk/programmes/m000mqn7
> "did they appreciate things purely for aesthetics and cultural relevance, and not utility?"
Many animals do, even sharing their sense of aesthetics with human taste. Just look at how flowers evolved to some form even we find pleasing without any skin in the game, or how animals that live in total dark (deep sea, for example) are atrociously ugly.
I think "create art" and "value art" are essentially equivalent in the context of the question we are pondering. You cannot argue they made art if there was none to appreciate it as art; art doesn't exist without an observer.
