Ain't evolution cool! Its never about "progress" in a certain direction, or increasing complexity, or any other "upward" trajectory we may think of. Its always about what works best for a given organism in its particular environmental circumstance: if a trait (such as lungs or skin to breathe through) works well enough to let you survive and make lots of babies, it may prevail and become the norm in your lineage; OTOH, if an existing trait (such as your newly evolved lungs) get in the way of surviving and procreating, natural selection might well get rid of it! Sometimes, your entire lineage goes with the trait - natural selection isn't really all that particular in that way. But if, like this frog, you have one troublesome big trait that is chafing against the filter of selection, and also have a backup system to perform the same physiological function, you may get away with losing the big new innovation!
These frogs apparently live in fast, cold, mountain streams rich enough in oxygen that they can get enough through their skins. Of course all amphibians can breathe through their skin to some extent, but the big innovation that allowed them to plant the tetrapod flag in the terrestrial realm in the first place, was of course lungs! Yet these little guys have given up on lungs, and, unable to re-invent more ancestral gills, reverted to the skin - and it'll be fascinating now to study how that skin has evolved in this species.
Meanwhile, the serendipitous discovery of this wonder must give pause to my friends who are against vivisection (for many good reasons, and some poor ones), and even against collecting specimens at all for museum collections or scientific study. And it becomes harder to argue with activists of that persuasion when it comes to rare new species. Yet, this amazing discovery would not have been possible if someone hadn't thought to look inside the body of the frog! Contrary to what the romantic Wordsworth wrote, I have to contend that we biologists do not always "murder to dissect"!