Now granted, things like insulin sensitivity also plays a role in how efficiently your body uses incoming carbs and where that glucose gets directed towards (remember, you muscle tissues is still made up of a good amount of glycogen which is glucose stored in muscle, and that glycogen can not in turn be stored in fat cells, that glycogen actually can not reenter the bloodstream, they can only be used to fuel muscles). Both body fat as well as muscles are forms of stored energy sources for the body (fatty acids stored in body fat, aminos stored in muscle). Exercise and proper protein intake will help spare muscles when there's not enough incoming calories to cover the metabolic cost of your activities and being alive. Also, ketosis is achieved when your liver glucose stores are depleted and your liver can only store X amount so it's not a bottomless container. If you don't have enough calories coming in, you don't have enough calories coming in. Ketosis is kind of a by-product of having a aggressive enough caloric deficit. I mean, you can cut out all carbs from your diet but if your caloric intake still ends up not hitting a deficit, you're not going to lose weight.
Think about this, if you had proper exercise and proper protein intake thus signalling your body to preserve muscle tissue whilst also being on a caloric deficit, even if you are eating carbs as per usual, if you have a caloric deficit in which at the end of the day, you did not take in enough calories to cover the metabolic cost of your activities as well as just being alive, where do you suppose the body is going to start getting those calories from? Calories doesn't just appear out of thin air.
But hey, some people believe in IF, some people believe in going into and staying in ketosis, at the end of the day, you see equally good results from all these different diet strategies, so, pick whichever one you can stick with I guess :P