No they don't rofl, all they are showing is MS difference between the player and the server. Since you are clueless and dumb i'll explain it to you AGAIN.
Because of MS difference a cast that started after a player/mob stopped moving will look like they are casting while moving.
Take this vod for example: https://www.twitch.tv/adlairo/v/84869358 skip to 58:40 and look at the warlock, notice how he is starting a cast while moving. Case closed it's MS difference between the server and player which makes it look like gargoyle is moving while casting.
Btw it doesn't matter how bugged dks are on the server or how bugged they will be, you still wont be good at it if you rely on these bugs to get your precious 2s rating.