The era they fought in MIGHT have a influence. In a present day 12 round bout, with all the health and safety of early stoppages, then Usyk wins. However a bout of 20 or so rounds and the rules of late 19th early 20th century with only stoppages happening if boxer parted from their consciousness, no stoppages on cuts etc likely, then that sort of bout is Jefferies arena. How much punishment could Jefferies take from the likes of Usyk and still comeback to win? The more obvious answer would be that Usyk wins, but the Boilermaker was honed from granite and steel, able to take inordinate amount of punishment and still prevail.
What is the point of debating who would win when the guys competed over 100 years apart? Nothing is the same not even the sport itself. Usyk wouldn't have the style he has now back then and Jefferies wouldn't have the style he had back then if he was born in the 21st century so what are we really asking?