Rows 13 and 14 on the internationally-configured 767-300 are indeed crew rest seats for international flights. They are sort of a hybrid between C and Y seats-- not as wide or as much pitch as the other C seats, but more pitch than Y seats, and with leg rests. (And there are video screens located in the armrests on those seats-- you should have asked the F/As how to access them.)

While I cannot condone an "irritated tone" or a rude "hrumph" from the F/As, they ARE only supposed to be used if there are no other Y seats available when customers check in. The reason it may have rubbed the F/As the wrong way if the CSRs allocated them improperly (which I assume to be the case since you mentioned the flight wasn't full) is because the C galley on this configuration is EXTREMELY cramped and difficult to work in for the level of service expected by management and by the customers, so it helps to be able to use those seats as an basically an extension of the C galley. Typically if those seats are not assigned to customers, during the meal service you will see the tray tables pulled out on all four of those seats and used as extra counter space.

Also even though they are technically Y seats domestically, the C service is provided "to maintain the integrity of the C cabin," (or some such wording that appears in the F/A manual) (i.e., a meal serice in this case as opposed to just beverages as served in Y) as they appear to be a part of the C cabin to the uninitiated.