WoW is the type of game where it can reatroactively change things when data is received and have it mostly be unnoticeable.
Your spell interruption example illustrates it perfectly - spells can be interrupted if the interrupt is within the 1-2s cast time of the spell. It's adjudicated on server side, where the server knows the spellcast start and sends feedback on the spell results (including if any interrupts happened). If there is minimal delay then it works without being noticed; however when I had significant packet loss, then the spellcasting would 'end' for me only after the response (including check if it was interrupted) was received, and it could be multiple seconds later.
The point is, that WoW can mostly function if the orders and feedback are received with delays, since if some opponent in reality(server) is somewhere else than I see, then it usually doesn't change anything - but for a rapid movement FPS game, that would change hits to misses which is the core of that game.
Your spell interruption example illustrates it perfectly - spells can be interrupted if the interrupt is within the 1-2s cast time of the spell. It's adjudicated on server side, where the server knows the spellcast start and sends feedback on the spell results (including if any interrupts happened). If there is minimal delay then it works without being noticed; however when I had significant packet loss, then the spellcasting would 'end' for me only after the response (including check if it was interrupted) was received, and it could be multiple seconds later.
The point is, that WoW can mostly function if the orders and feedback are received with delays, since if some opponent in reality(server) is somewhere else than I see, then it usually doesn't change anything - but for a rapid movement FPS game, that would change hits to misses which is the core of that game.