Because these hybrids would contain mtDNA from their human female line. Neanderthal mtDNA could only be passed down by Neanderthal females.

And because none of those are found in any modern human populations, we can conclude no humans today are descended from female Neanderthals. Though whether hybridized descendants from male-sapiens female-Neanderthal pairings never existed, or they did exist for some time then eventually went extinct, we cannot currently say with certainty.

Strictly speaking we don't know that. It may always turn up an extremely rare Y or mtdna variant which was thought to be extinct. Ötzi's mt like was thought to be extinct (Wikipedia page even still says so) but very recently a North African man took a full mtdna test and it turned out he had the same. That could happen with neanderthal variants too for all we know.

> we can conclude no humans today are descended from female Neanderthals.

that looks worded wrong, strictly speaking. if there's a male neanderthal ancestor, then he very likely has a neanderthal mom or grandma or ... great^N grandma for some N.