Ultimately, what matters is that the WR is where the QB is expecting him to be. It also means being in the right place in relation to other routes so that the movement you're trying to get from the defense leaves someone open. There are so many things that go into it... so is there something specific you're talking about when you say Holker was a great route runner while Rex wasn't?

Running the correct route (running the route for the play that was called i.e. - post vs fly, etc)
Having good technique in breaks (crisp breaks, not a lot of rounding, etc)
Breaks coming at the right time or location (breaking at 7 yards, rather than 6 or 8, if that's what the play calls for)
Bending/adjusting a route based on defense (John Beck talked a lot about this in his videos - a post might be called, but the receiver has to adjust it based on the defense - make it skinnier, make it flatter, etc)
Sitting down in a hole in the zone, vs running through vs man, etc.

Again, all those things and more go into route running, so what aspects made Holker so good, or what made Rex struggle? What things does Keanu do well, and what does he have to work on?