Neither really has anything to do with one or the other, but they do supplement each other which is why a lot of people mention them together.
Going with a ketogenic diet actually makes eating only one meal a day much easier. If you’re on a ketogenic diet, it means that you’re consuming an extremely small amount of carbohydrates per day—talking like less than 20g. Instead of getting your energy from carbs, you’d be getting about 70% of your energy from fat, and about 20% from protein.
Now, the point of this extremely low carb, high fat diet is to put your body into a state of ketosis, which means that it begins using fat as its main source of energy as opposed to carbohydrates. The benefit of this is that you obviously stop storing fat, but the other benefit is why it goes well with OMAD.
The other benefit is that when you don’t consume carbohydrates, your blood sugar levels stay the same, as opposed to spiking and then dropping below average. When your blood sugar spikes and drops, you feel lethargic and hungry, and you feel the need to eat something. If your blood sugar never gets the chance to spike, then you’re going to feel full of energy as your body continues using your own fat for fuel, meaning it makes it easier to eat only once a day.
This is why they go so well together.