I think if you read the explanation, he ranks weeks like that so low because he is figuring on a first and likely one-time visiting family that will want to see and do nearly everything and the thought is that the abbreviated park hours and refurbs will get in the way of their accomplishing that. So the ranking isn't based on crowd level really.
We've gone during marathon week twice and never experienced anything other than minor traffic issues. And of course, we've never ridden Splash on those trips, since that's the traditional time frame for its refurb.
Agree with DavidNYC above though, that this shouldn't deter you. Our first visit was during this time frame and at that time, we had no plans to go back (obviously that changed, lol!). Even so, we didn't feel like we were missing anything. It's impossible to see and do everything anyway, and I feel like one's enjoyment would be diminished by attempting to do it all in one visit, no matter what time of year it was.