High in sugar so yes high in carbs.

I don't know anything about this web site. Just found it in a quick search. But it claims 20 grams carbs in a slice or 10 watermelon balls is 9 carbs.    Carbs in watermelon.

Could slip it in once in a while in a low carb 100-120 grams a day diet. But definitely not in keto unless that was the only carbs a person had all day. Even then it might or might not kick a person out of ketosis, depending on how metabolically healthy they are, temporarily at least if eaten all at once. It is generally estimated that for most people to be in ketosis they have to limit carbs to under 50 grams per day and if a person is hyperinsulinemia (diabetic or pre-diabetic) under about 20-25 grams.

Better fruit choices are strawberries, blueberries, blackberries or raspberries. They are the lowest sugar content of the fruits other than avocado which is mostly fat and is not sweet at all.

A little once in a while is not going to kill you but it sure will not help any weight loss or ketosis goals.

John