It should be noted that the fossil is not a whole dinosaur tail, but rather only a small piece of a tiny feathered tail measuring about 1.4 inches in length and containing 8 vertebrae, each about the size of a grain of rice!
Rhetorically downplaying the significance of even the smallest facts, data, and evidences?
It is estimated that this would make this presumed relative of T. rex about the size of a sparrow. The tail piece is undoubtedly covered with tiny feathers that are essentially identical to those of modern birds, but is this in fact a dinosaur tail rather than a bird tail?
Again downplaying the significance of the find by saying it's too small. There's also a hint at subterfuge in this quote. "Presumed relative of T. rex": Xing et. al. never claim it is a first or second cousin of the T. rex. The implied argument is a straw-man. "Essentially identical to those of modern birds" is likewise not a claim of Xing's but an uncritical, undocumented statement on the part of Dr. Menton. In fact, Xing et. al. claim quite the opposite. I quote from their "Supplemental Information" pages 10-11:
Additional details on taxonomic placement
There are several lines of evidence pertaining to the systematic position of the specimen. First, based on the preserved length of the tail and available measurements of the preserved caudal vertebrae, we estimate that a complete caudal series is likely to comprise more than 25 caudal vertebrae, which indicates the specimen more likely to be a non-avialan theropod as there is only one known avian species (i.e., Jeholornis) with more than 25 caudal vertebrae. Second, there is a distinctive ventral groove on the caudal centra of the specimen, which is widely distributed among non-avialan theropods but which has yet to be reported in avialans (though the possibility of its presence in the two known long-tailed birds Archaeopteryx and Jeholornis cannot be excluded). Third, all preserved tail feathers lack closed vanes, a defining character of flight feathers within the Pennaraptora. This suggests that the new specimen is probably more stemward than known pennaraptorans, a systematic inference consistent with osteological criteria. Furthermore, the presence of open vanes (indicated by the barb arrangements on either side of the rachis and the presence of barbules) suggests that the specimen is more crownward than compsognathids and tyrannosauroids (and maybe even ornithomimosaurs, which have been suggested to have some kind of pennaceous feathers). Taken together, the available osteological and integumentary features suggest that the specimen is probably a maniraptoran more crownward than compsognathids and tyrannosauroids (maybe even ornithomimosaurs) and perhaps more stemward than oviraptorosaurs.
However, the juvenile nature of the specimen might weaken this systematic hypothesis. The extremely small size of the specimen suggests that it is a juvenile. The longest measurable caudal is about 4 mm and we believe it is from the posterior part of the caudal series (because the succeeding caudals start to decrease the size). All known adult theropods including the two known long-tailed birds Archaeopteryx and Jeholornis have much longer posterior caudal vertebrae (e.g., 17 mm in J. palmapenis [S11]). If the specimen is a juvenile maniraptoran, it is possible that it may exhibit less complex feathers as a result of its ontogenetic stage. However, there are three lines of evidence that suggest the plumage is representative of the adult morphology regardless of the juvenile nature of the specimen. First, Similicaudipteryx fossils show that even juveniles of non-avialan theropods have pennaceous feathers (the late juvenile individual has typical flight feathers with closed vanes). Second, in extant birds the flight feathers (including tail feathers) have closed vanes with pennaceous barbules once they have replaced the plumulaceous neoptile feathers. Third, there is no evidence from modern feathers to support the idea that the largely symmetrical and somewhat pennaceous barbules seen in DIP-V-15103 might transform into a different type of pennaceous barbules (i.e., asymmetrical pennaceous barbules capable of forming a flight feather with closed vanes) during a later moult.