Man, errors don't really tell you very much about how good somebody is at defense anyway.
You could put my 40 year old bad knees that haven't been on a field in 20 years at first base for the Cards right now, and if by some miracle I went the rest of the year without making a single error, I guarantee you I would STILL be easily the worst defender of all time. I'd be missing scoops anybody could make, I'd almost never make a great play on a tough scoop, I'd be messing up plays on bunts, misplaying tricky foul balls, throwing to the wrong base, be in the wrong place on cutoffs sometimes, make poor throws on the front of DPs, slow to cover my base on tricky plays, on and on. It has to be stuff like that is what has these guys here saying this guy might be the worst defender EVER.
For guys that watch him every day to be saying he might be the worst ever, it must be some version of all of the above- just kinda bad at everything, almost all the time. The organization wants to win, and apparently his great bat can't outweigh the innings his poor defense is routinely extending for them. That or it's some kind of personal thing we don't know about in addition to the poor defense.