Lack of End Game Content

[Kaylessa]

Content is also a noun that can mean "the things that are held or included in something" or "the amount of a particular constituent occurring in a substance." It means something that is inside or part of something.

That is correct. Now apply the modifier "endgame" to content and you get "max level things that are included in something". The reward you get is the max level content, farming that reward is the max level activity, running Sastasha doesn't fit the definition of "max level things that are included in FFXIV" and so that is not endgame content.