I've got a theory (for real just hear me out on this)

Due to the 10 Hour Burst Man's NPC-like movement, I believe that he used to be a Dummy, maybe a Borrow Dummy, an NPC that can have any item.
Any item.
10HBM has a large variety of items at his disposal (Twilight, Darkheart, Grand Volley, Strong Left, Star Platinum, etc.), so he could be a prototype of the Borrow Dummy that could use any item and not just melees, but could borrow infinitely (hence his large variety of items). The very first item they tested this new dummy on with was a bottle of 10 Hour Burst, likely in abandonedfacility (which is why it is abandoned), and it kept its new form across lobbies and maps. It borrowed so much, and eventually cast away his old identity of a failed Borrow Dummy to the 10 Hour Burst Man we know and hate. The boss fight is at the end of his life, when the Asylumers finally decide to stand up to him.

if the lore is already confirmed for 10hbm tell me if this is at least close or not