Put it this way, what is the difference between an object imbued with the force and radiates the force(idk, like a Sith talisman or something like that) and a nexus, which also radiates the force and can affect those in it?
Sorry if I was pretty ignorant on this matter and wasn't aware of something basic here, but I always assumed the two were similar from the start and didn't think much of it.
True nexuses are defined as places/objects where "the force flows freely, like a torrent, flooding the surrounding area with power" -- and in true nexuses, the force is "exponentially amplified".
Kenobi's rookie lightsaber hardly qualifies as such. It may have been mildly strong/unique in the force, but that doesn't make it a literal nexus.