Admittedly, Irving is not playing against the best offensive players most of the time. CBS wisely has him off ball on the opposition's wing, normally weakside of the court.
In that role, Irving has thrived as a defender with charges, strips, and rebounds.
It's ok to recognize that he is not on-ball against the other team's best player most of the time. But it's also ok to recognize that he is thriving on defense where he is.
That said, Irving has also had a bunch of big plays on ball against the opposition's best players. That charge that he drew on Butler against the Sixers is an example of this. He is not a consistent high-level on-ball defender, but he's always had the ability to make timely, winning defensive plays.