You are correct in saying that running a 10k in x isn't necessary for the marathon as long as you are able to run a 10k in x. I realized this after my first post, before the others.
I personally believe he needs to be able to run a faster 10k. After doing a bit of research, however, this hypothesis has almost zero backing. according to alltime-athletics, a 2:07:56 marathon performance ties for the 1700th fastest ever run. (we must note that there are many more professional 10ks run per year than marathons) The 1700th fastest 10000m ever run is a three way tie at 27:43.30. Richtman's 10k PR comes from a cross country race- 28:0x. Since 10ks are more common than marathons and Richtman is probably better at the marathon than 10k, my theory says that the 1700th fastest 10000m should be faster than it is. So it looks like I;m wrong. By the way, a 2:04:59 marathon is the 211th fastest time ever run, and the 211th fastest 10000m is 27:03 .... these numbers don't correlate, but at least they are numbers.
tl;dr - the marathon is probably Richtman's best event, and to run a faster marathon he must inevitably be more fit, which does not necessarily mean he needs to run a 10k so-many seconds faster. I was incorrect.