The other side of the coin. . .
Extensions from an existing K&T branch circuit have a history of being normal wiring practice. Depending on local ordinance, they may still be legal to create. The NEC still describes how to do it without a j-box.
300.16 includes "fitting" as well as "box". If the open air splice you are looking at has a "monkey face" transition fitting on the end of the NM cable sheath, then the splices to the existing K&T are permitted without a box exclosing them.
