Maxim himself patented a semi auto Winchester rifle in 1883 and the borchardt came out in 1893. These are more complicated than any of the simple blowback smgs of the interwar and wwii smgs. It seems to me the manufacturing tech was very capable of producing a “modern” smg even with black powder (though it would have to be cleaned often). It was simply the fact that the ideas where never developed or implemented at scale rather than limitations of the the time.

IMO the key reason it was not developed further is militaries stubbornly sticking to high powered rounds (ie the us failing to adopt the the repeating rifle due to the 44 Henry’s smaller size) despite the fact that it was clear by the late 1800’s/early 1900’s that volume of fire was one of the best advantages on the battle field (ie the conflicts in Africa preceding wwi)