There are other important factors in a WR:

How well can they catch?
How well do they react to the defense?
How coachable are they?
etc...

Not sure where Barden ranks there.

No, I'm not misunderstanding the article. The author is completely offbase in his attempt to use this differential as a tool for assessing physical ability. All his differential tells you is how a person's 40 dash time is relative to his shuttle time, which is a useless metric.

For an example of how asinine the author's contention is, note that he singles out Ron Brace as shining for a great differential. The author even rates him as having the best "lateral agility differential" of all defensive linemen. However, if you look at the Combine results, Ron Brace was terrible, and Brace even acknowledged that it was his worst performance ever in the 40! What made his differential "great" was that he was absolutely horrible in the dash (a last place 5.50 time) whereas he was only in the worst 25% for his shuttle time. Brace was also in the worst 10% for his 3-cone shuttle time. He's not only track slow, but he's well below average in his lateral agility too.

The author is simply wrong. If we want indicators of lateral quickness and agility, we need to look at the discrete shuttle times and the 3-cone drill times relative to their peers.