It's taken me awhile to wrap my head around what you're asking and trying to think of what these people were thinking when they made this claim.
CICO is not simple.
For example, if you restrict CI, CO does not necessarily remain constant. Think starvation mode. You cut back by 500 calories a day, your body will eventually decrease BMR by 500. The result is a slowed metabolism and no long term weight loss.
Conversely, if you increase your CI, and keep insulin low (which is the hormone that tells your body to store excess energy), then your body could increase your BMR to burn off the excess calories. Basically you'd be increasing your metabolism (you can look up this process for fixing a broken metabolism). You could also do this through exercise. There are a lot of things that go into this equation.
Maybe that latter example is what they meant. I have no idea. Generally speaking, CICO is correct, and you CAN gain on Keto.
Years of calorie reduction diets that killed my metabolism, followed by bouts of overeating, have made me this fat. I have to lose the weight, then fix my metabolism. I will attack this with lifting weights, HIIT, and keeping my insulin low through fasting. When the time comes, I'll eat carbs on workout days, fast on non-workout days.