the airline (XL) does not now include in flight meals. the brochure quite clearly states that the price includes them
XL still provide meals on their planes and as an in flight meal was included as part of your booking conditions you should still be provided with a meal on board.
Olympic can't pass the buck and say that it is the airline not providing meals, Xl provided a meal and it is up to the tour operator (Olympic) to pay for it now if required, they have accepted your booking under these T&C's.

Having said that I have just read on the XL website that certain flights (Those flight # ending in V) are now classed as low-cost and no meal is provided, however you can purchase food.
Info re this change
http://www.xl.com/Customer_Info/Additional_Short.asp
This page
http://www.xl.com/Customer_Info/Popup_Meals.asp shows the meals / snacks available so if your flight doesn't have a meal Olympic should provide a voucher or reimburse you for the cost of food.
I would contact Olympic and tell them you wish a meal on board as per your booking.
Having read what you have said I fully expect to have to do the same with Olympic for a holiday in Symi in October.
I noticed on their website yesterday that in flight meals were now not included the same as yourself, however they were included when we booked, so it should only be those customers that have booked under the new T&C's i.e. no meal that shouldn't receive an in-flight meal.
I have checked our flight on the XL website for the above holiday and they still show as meals being given onboard as part of the flight so it might be worth your while checking the XL website
http://www.xl.com/ for your flight number to see if it ends in V, if not you should still get a meal provided.
Please let us know how you get on.
Graham