sure and certainly between species neuron density may vary wildly resulting in some smaller being smarter than other larger. The researchers wisely made claims on what they have, and that does not include neuron density over eons within our species nor paleo intelligence tests in general.

But being a poster child of typical myself I can't help but suspect the most parsimonious result is (statistically) larger cranial capacity affords higher intelligence potential all other things e.g, neuron density, being equal.

Half a moment of cherry-picking web search results for "cranial capacity and intelligence in mammals" turns up "External Measures of Cognition" [1] which "surprise!" agrees.

[1] https://www.ncbi.nlm.nih.gov/pmc/articles/PMC3207484/