The airline is definitely liable for the contents as well but that's not what the answer you received is about. For example, you can get compensated if your luggage is lost completely and not only for the price of a new bag. But airlines do not have to accept liability for valuable items (unless you declared them as such) or improperly packed, fragile or perishable items. And apparently they are arguing that they are not responsible in this case simply because marmelade is food.
For you might be surprised by what counts as “normal treatment” for hold luggage. It all comes down to how you packed it, the mere fact the glass was broken really does not prove anything out of the ordinary happened. Next time you travel, fully expect your luggage to be thrown around, turned upside down several times, fall some distance from one conveyor belt to the next, hit other pieces of luggage, be stacked on a cart with four-five heavy bags on top of it, etc.
Beyond that, I don't know whether the details of easy jet's conditions of carriage or their application in this case would hold up in court but my personal experience with low-cost airlines is that they will keep you in a holding pattern with boilerplate answers and are even less helpful than legacy airlines (which could offer you something, not because they are liable but simply as a commercial gesture). The problem is that escalating things beyond an email (e.g. hiring a lawyer) is likely to be too costly to make sense, even if you had a strong case, which you do not.
One last option is to turn to social media. Airlines are sometimes more responsive that way, to avoid public shaming.
Incidentally, and without giving too much weight to some poorly translated boilerplate text, “unabhängig von den Umständen” presumably refers to the things listed in the first sentence, i.e. valuable items, not to everything you could have in your bag.