If you’re churning anything while running, that’s energy being diverted to churning.

Yes and no. If you're putting the same amount of energy in to move more mass, yes, some of that energy is diverted away from some aspect(s) of your mechanical motion into the cream. However, you can also just put in marginally more energy to compensate. A quart of cream weighs ~1lb. If you're a 120 lb runner, you've increased your weight by 0.8%, so you only need to increase your energy output by that same margin. That's not a big ask. And it gets smaller the heavier you are.

but it uses different muscles.

I mean... not really? Presumably, you're carrying this cream close to your body somewhere on your waist or torso, like in a pocket, pouch, tucked into a sports bra, or something similar. That means it's going to be relatively close to your center of gravity. And, again, we're talking ~1lb here for a quart. So it's not going to move your center of gravity significantly between its mass relative to yours, as well as it's proximity to your center of gravity. 1 lb is not significant. Your can gain a pound rating pasta right. And runners typically lose 1-4 lbs of water per hour on a run from sweat and respiration anyway.

but if your goal is to improve running technique, pace and speed, there are better ways to do it.>

I think the goal is the novelty of making butter without really trying to, or simply multi-tasking.