If you compare the art and technology of Homo sapiens 50,000 years ago vs 50 years ago... it's clear that 50,000 years ago gives no inkling of Homo sapiens' intelligence.

So how could we possibly infer that Neanderthals were less intelligent? How could we possibly separate their intelligence from their state of cultural and technology evolution?

Edit: indeed, a quick Google search reveals that the idea that Neanderthals were less intelligent than humans has no real evidence, that it's now considered to be an outdated/discredited view. So wherever you seem to be getting your facts and conclusions from, appears to be wrong.