Yeah jump-rope is great...but, if you can jump rope for 20-30 straight with only very short breaks then this is they ULTIMATE CARIO workout (this is compareble to non stop sprints)
Gotta say that it's too much for me...45 min on the bike are way easier
If you are in ketosis then jerking off burns fat hehe . When on a normal diet there are arguements as to what the best heart rate for fat burning is. On a keto diet no arguement is needed as anything you do will burn fat as you are using only fat for fuel.
The answer to the question though is jump rope is a very good cardio workout as it is very demanding. I would say if you can truly do 20-30 minutes than do go for it as you'll burn more calories than low intensity cardio and it will have more of a benefit to your heart. Low intensity cardio doesn't really get you in to aerobic shape it only burns calories. Higher intensity cardio will burn calories and give your cardiovascular system a workout.
Jump roping is a good way to do interval training as well. Go full out for 30 secs then slow down for a min and repeat. I've done this in the past but I'm a girly man when it comes to cardio, I'd rather fast walk on a treadmill and watch tv .