I'd go with that definition. It's just a standard, a means of comparison. Specifically meant to exclude the categories you cited, if only for purpose of discussion. To quibble, it does include the pack itself (not just "everything in it.")
That it changes by season (or other reasons) should also be no surprise. Earl or late season calls for heavier or more layers. Mid summer, cut back on rain gear. Pack size and contents might change depending on remoteness or length or purpose of the trip. Etc.