No definitive answer to your question, but some rambling thoughts...
Charles Musser refers to a Lumière film titled A FRIENDLY BOXING MATCH in his
The Emergence of Cinema: The American Screen to 1907, and on p.599 lists it as another title for BOXEURS. Although he doesn't explicitly state any reason for this, his decision
may have been based on the very early playdates associated with the film (which are
seemingly too early for it to have been BOXEURS EN TONNEAUX).
I haven't seen any primary sources that employ the title A FRIENDLY BOXING MATCH. Rather, the earliest Lumière screenings by B.F. Keith during June/July 1896 included a film titled A FRIENDLY BOXING
BOUT. The date is so very early that BOXEURS seems a likely option; however, like you, I struggle to see anything obviously 'friendly' about the fight in this short. The Fall 1897 Maguire & Baucus catalog would go on to include BOXEURS under the title BOXING MATCH, referring to it as 'A lively boxing
bout between two first class artists.'
A further possibility might be Lumière #114, GÉANT ET NAIN (
https://catalogue-lumiere.com/geant-et-nain/), which was certainly produced in time to have been circulating during June/July 1896. The Fall 1897 Maguire & Baucus catalog includes this one without an accompanying description under the title THE GIANT AND THE DWARF (CLOWNS).
As for BOXEURS EN TONNEAUX, it likewise appears in the latter late-1897 catalog without a description, as A BURLESQUE BOXING
BOUT. The Lumiere number for it, #104, is sufficiently low... that I can't help wondering whether there might have been an earlier version of it than that shot
circa Winter 1896. Certainly there have been a number of Lumières from 1896/97 of which three or even four variant versions, shot months apart and on different sets or locations, have turned up in archives. [One clear example is Lumière #116, the knockabout comedy JURY DE PEINTURE, listed as a Sept. 1897 title, but known to have existed in an earlier incarnation that played in France as early as July 1896; and at
Mostly Lost 6 we saw yet a third version shot on a totally different set that was in use around Sept. 1897 - potentially an export version that ended up in the Maguire & Baucus catalog as A JURY OF PAINTERS. In other words: don't let the hyperorganized veneer of the Lumière Catalogue lull you into thinking that it really has all eventualities or all of the company's output completely covered... as you know well enough from previous 'uncataloged' extant prints!]

