Why/how do people have Ringed City weapons as early as Farron Keep?
As there are no level limits for game progression, it is possible to get to DLC on any level. And while knowing weapon requirements, it is possible to level up according to requirements. E.g., while starting as Deprived, it is possible to meet requirements on level 23.
Therefore, it is possible to get DLC weapon on 23-25 lvl and roll in PVP with it.