Though Trident of Poseidon broke all the curse, the return of Davy Jones is still a plot hole. This end credit sequence can be a setup for the sixth film and given that Disney is counting on another sequel, this appears to be true.
This possible plot-hole might be explained in the sixth film and Davy Jones might be main antagonist in it. Till then, this is pure speculation and nothing else.
From an interview of director Joachim Rønning given to Cinema Blend
It's difficult to ever know. I think that there's always... this is the beginning of the finale. [laughs] It depends on how you look at it. But I never take anything for granted, and I love the franchise. These kinds of movies, they remind me of the kind of movies I grew up with, those kind of adventure movies that made me want to become a filmmaker. So for me to be able to be a part of this and direct a Pirates of the Caribbean is a dream come true. But I don't know if it's the last one, or if they'll make another ten of them -- I don't know! But I hope so!
Producer Jerry Bruckheimer also hinted for sixth movie.