If he's an arms dealer, battling extradition, and trying to save his skin, it is definitely material. It implies a motive to lie, to protect himself from jail or extradition.
Everything he says might still be true. But at this point, if my choices are: (1) Trump's FBI and DOJ decided he was making it up or just not credible enough to believe or (2) there is a vast conspiracy between the FBI, DOJ, federal judges, etc. and that he at one time had incriminating documents, but conveniently does not anymore because he gave his only copy to the people seeking his extradition. . . well, Occam's Razor and all.
I'll admit, though, I'm a bit confused by the story linked. Not a model of clarity, there, in laying out the scheme that involved CEFC paying to use the Biden name to push its belt and road campaigns in other nations in 2017, after he left the VP position. Is that illegal?