Farah is the best runner in the world at 5k to 10k and is the favorite in any race he runs at those distances. He is a very good half marathoner, but is not the favorite against the best fields. Kamworor (and Karoki) beat him pretty soundly at Cardiff, for example. It's possible that Farah hasn't shown the same ability at the half as he has at the 5k/10k because of his training. Though good 10k shape typically means good half shape, Farah might do a lot better at that distance under marathon training. I think it's premature to dismiss Farah at the marathon for the same reason, though he might be past his prime at this point to contend with the best due to age, even if he has the talent.
I agree that Kipchoge is mostly being diplomatic with his comments re: Farah, though I think he certainly will be watching him. The simple fact is that Kipchoge is on another level and doesn't really "worry" about any of his competitors.