I have no idea what you're talking about.
The defensive EPM is just another impact metric. This particular stat doesn't tell us who the "best" or "worst" defender in the league is (no such stat exists). It tells us a player's offensive/defensive/net impact within their own team.
No it doesn't. You're looking at "Estimated EPM", which is a prediction of what a player's defensive impact on their team will be in any given game based upon their entire career.
Keyonte's individual offensive impact on the Jazz is well above average this season.
The pic you posted used entire career data, not just "this season".
The problem is that his individual defensive impact is the worst in the NBA. By a mile. That means that overall, he's still a negative player for the Jazz.
The eye test says Key is one of the worst guard defenders I've ever seen in the league and the stats bear it out.
The eye test? He isn't even getting targeted. We have no rim protection. Here is ACTUAL EPM, based on this season alone:

There's an easy conclusion to make here, and it isn't "WCJ, Collier, and Keyonte are all historically bad defenders."
It is "Playing most of your minutes with Kevin Love at center will wreck your team's defense."