What's the difference between pickling and fermentation in cabbage?
It is no different in cabbage than other vegetables.

Pickling is preserving a food with a salt brine.
Fermenting is to allow tasty bacteria to work in a food to change its flavor and often to acidify it.

They are not the same thing but there is overlap.

Some pickled foods are also fermented. The salt inhibits bad bacteria. These tend to have more complex flavor and be more sour. Cucumber pickles, for example, can be made in just a brine or fermented.

Kimchi and sauerkraut are both fermented. The cabbage is packed in salt and let age until sour. Technically they are pickled but they aren't usually referred to that way because they are their own thing.

I've never seen a coleslaw recipe that was fermented. It also isn't really pickled because it doesn't have enough salt in it and it isn't aged. Coleslaw is usually made with just an acidic dressing.