From the abstract: "This new bird, however, has a normal-sized major digit claw, as in other basal birds, which was probably regained independently as Confuciusornithidae evolved, based on our phylogenetic study. Unfortunately, the biological significance of this trait is unclear owing to a lack of analogues in modern birds (manual claws are completely lost in adults)."
It's a common misconception that all modern adult birds lack manual claws. Actually, many extant birds possess manual claws, even as adults - and this has been known for a very long time (e.g., Heilmann, 1926; Fisher, 1940). These wing claws are distinct from the wing spurs, clubs, knobs etc of certain birds that are used for defense or combat. This topic has been dealt with by Darren Naish:
"While itâs well known that the fossil birds of the Mesozoic often
had clawed hands, it doesnât seem to be widely known that hand claws are
widespread and in fact wholly normal in extant birds. Theyâre not an
anachronism unique to the Hoatzin _Opisthocomus hoazin_..."
These remnant wing claws in extant adult birds appear to be useless. This might also be the case for the wing claws of confuciusornithids, at least for the 2nd and 3rd fingers. These two digits were likely bound together, as in _Anchiornis_, enantiornithines, etc.Â
Thus, the relative size of the claw on the major digit (= digit II) of confusiusornithids, which differs between _Yangavis_ and other confuciusornithids, probably has no functional significance. The large claw on the first digit, which lay completely outside the wing, might have functioned in defense/combat (there's no evidence that confuciusornithids had an alula).
Incidentally, the genus _Yangavis_, named in honor of Prof. Zhongjian Yang (= C.C. Young), effectively has the same meaning as another Mesozoic bird,
_Zhongjianornis_.