I occurs to me that the answer to a question such as this is that some classic courses do stand up better to even tour caliber players despite lack of length. But why?
Firstly, length alone, the type of length cited here, even 7400 yds, has basically been proven to not be a problem for tour caliber golfers although it certainly would be for the rest of us.
So why would a shorter course be more of a problem for them to score well on? My guess would be in some classic course cases the greens would be the answer.
Many classic courses have greens that are more complex for tour pros or anyone else to play despite being shorter total distance-wise.
Some older classic courses have greens that really weren't built for the speeds that they're running today, as are many of the greens on longer more modern courses and consequently hitting them properly, recovering to them or birdied them is obviously more diffiicult than what they find on the newer, much longer modern courses.
That's probably the answer.