I'm not sure that's exactly true. And even if it is true, you can't stop coaches and runners from trying to compare times from course to course.
I think if you were to assemble enough data, you could probably develop formulas that compare performances on different cross-country courses. The length of the course, while important, wouldn't be a factor in the equation, however.
In Illinois, the state meet course has been in the same place- Detweiller Park in Peoria- and essentially has been the same course since 1971. Other courses are rated (informally and unscientifically) as being x seconds faster or slower "than Detweiller".
The problem is that while Detweiller has stayed pretty much the same, other courses haven't. Park districts build fitness centers or turn start/finish areas into revenue producing soccer fields. And if the other courses change every couple of years, you can't assemble enough information to develop a robust conversion formula.