"He held his own defensively about as well as any other PG in the league." Meaning, except for a handful of the league's premier PG defenders and absolute worst defenders, every starting PG is going to give up to opposing PGs about the same ratio of big games, medium games, and little games. Isaiah's '14 game log shows that same ratio, more or less. (I didn't understand your correlation/causation point about game logs...but to be fair, I don't think you did, either.) He's not great defensively, he's not bad. That is the original claim, by the way, that Isaiah is too undersized to handle other starting points every night. His game log the last season he started, last year, says otherwise. His opposing PER says otherwise.* Even the DWS you cited have him only slightly trailing the middle of the starting PG pack in '14. You now seem to want to crop and reframe my opinion of Isaiah's defense into "He's good!" where I've only been arguing for his averageness and not-badness.
*TP for that, I don't really know where to look for stats like that.