Lots of guessing here, but it's more "complicated" than it seems.
I didn't watch the video, but sure, maybe Kipchoge "moves to roads" when it rains. Kipchoge also has a car and the ability to go where ever he wants (ie. Eldoret) to get his training in. Or money to take a taxi etc. The vast majority of the runners in Iten don't have running water, better yet cars, so they are stuck in Iten where there is only one paved road. And running on pavement for them is very, very rare. It sounds ridiculous, I know. Generally, only the race walkers use the paved road in Iten... not to mention, it's cluttered with donkeys, taxis, trucks, bicycles and it's understood that pavement is bad for your body.
Secondly, there's a lot of "follow the leader" going on. If Wilson Kipsang says he's going to stay inside when it rains, everyone else does the same. And most people don't then hit up YouTube and look for a strength workout like we do in the USA to compensate... they mainly just sit there. Play cards, sleep, lay on the bed, socialize.. it's pretty simple.
Last, there simply isn't medical attention readily available. So, training 12 hours a week for years on end with no money in return, which is the case for the majority of runners in Kenya, only to go out and get sick in the cold or slip in the mud and twist and ankle... bad idea.
While there is a shocking amount of discipline in Kenya, there is an equally shocking amount of relaxing going on. One thing Kenyans know well is hard work.. anther thing they are great at is not stressing--about anything really. If it's raining, justy chill out and move onto the next session. Simple as that. "There's always another day!"