This is an accurate assessment by Sage C.
Sub 2:40 in a road marathon requires specificity. Sub 2:30 requires more specificity. Faster than that requires even more.
People like Carpenter with very high VO2 max can only go so fast without specific training.
Genetics and biomechanics also come into play when running really fast. Not everyone has what it takes.
The mental aspect is huge. Is KJ willing to sacrifice what he really wants to do in order to pound flat paved roads day in and day out? Would he want to put in an honest year or two of specific training, knowing that its a long shot that he might crack the top 20, even if it all works out and he has a good day?
This is a typical speculative LRC thread; since so and so did X for this, can they run Y for that? Lance, Walmsley, ironmen, Jorgensen...
The correct answer is if you ran X for this is what you did for X. You have to actually run Y to claim that as an accomplishment.