The argument is that he could Kevin Pillar himself to a 2 WAR season. He's fast, he gets good reads on the ball in CF, and he clunks the f*** outta things out there when he's under pressure. He's got the "catching the ball" version of the yips. If he could figure out when to close his glove, he could be a VERY solid CF (something we don't really have, outside of maybe Davis). I don't think he's going to be a star or anything, but he has the tools to be an excellent CF, with an 85 wRC+ kinda bat, and can be a good tool as the 26th man as a pinch runner/defensive replacement type.
Puig is absolutely an upgrade over Alford, I'm not trying to argue that he's not, I just don't think that Puig has any more great seasons in him, and he's probably not enough of an upgrade to justify the cost, or the ******** that he brings with him.