It’s basically bullshit. They did a DNA test on an item with no demonstrable connection to the case (and which may not even have yet been manufactured in 1888), and then found mtDNA that’s compatible with modern relatives of Kosminski. It’s GIGO. You can’t do DNA testing on a century old, contaminated artifact of unknown provenance and hope to get a meaningful result out of it.
As I’ve said elsewhere, this would be like some family presenting a jacket they say their long dead relative took from a Zodiac crime scene, a claim for which there is literally no evidence. Then you do DNA testing and find that there’s DNA on it that matches relatives of one of the known suspects, but also millions and millions of other people. Is that case closed? Hell no.
This case gets solved every few months, and always using very flimsy arguments. I am sure that will still be happening 50 years from now too.