CPJ is not omniscient. He is fully capable, and shown, that he can make mistakes in judging who gives us the best chance to win. And even if Skov gives us a marginally better chance to win this year, getting Marshall experience now will give us a better chance to win next year. And so Johnson absolutely needs to weigh whatever slight benefit playing Skov might bring this year, against the benefit of getting Marshall experience will bring in the following years. And keep in mind that through 4 games there is no reason to believe that playing Marshall more would have lead to fewer wins.
And you may not want to spend time training up a freshman, but I don't want to spend time training up a senior who, by the time he's trained, won't be here. Unless Skov is clearly the superior back, which I see no reason to think he is, then Marshall, or Allen, should be playing.