KKG refers to the blood of an Uchiha which is why Itachi stated both KKG and Sharingan. If Sharingan was referred to as KKG then he would be saying the same thing twice. DB clears that up as well.
It's already been said genjutsu can be broken with kai release, Tsukuyomi is a genjutsu and therefore follows the same rule, it just requires a user with very high level chakra and chakra control to break it.
Itachi never faced someone with the same level of chakra as him until he faced Sasuke, so his perspective on the recommendations to break Tsukuyomi were severely limited. His reaction towards Sasuke breaking it prove that he did not know the claims he made about his dojutsu could be contradicted.
The manga explained how genjutsu works. A person gets trapped in an illusion when their chakra is being controlled, to break through the illusion the victim needs chakra of an equivalent level as the genjutsu user to break free of their control, like how you need the same muscle strength as someone to break out of their control.
When Sasuke broke out of Tsukuyomi it's because he release CS level 2 chakra, which has been confirmed to be of equivalent strength as his chakra after he awakened Mangekyo. He essentially released a chakra capable of matching Itachi's and as a result broke free, it had nothing to do with his Sharingan or DNA but rather his level of chakra.
We've seen other techniques that work in the exact same way as Sharingan genjutsu. Chakra rods and Edo tesnei both control the victim like genutsu by controlling the victim's chakra flow, but both techniques can be broken if the level of chakra release is high enough.
Naruto broke free of Nagato's chakra rod during their conversation thanks to Kurama like how perfect Jin break free from genjutsu with bijuu, likewise Hashirama and Madara both broke out Kabuto's control due to their superior chakra, while other shinobi with lesser chakra remained under Edo's control, including Itachi.
So if an opponent has at least the same level of chakra strength as Itachi they should be able to break his MS genjutsu. Because the stronger the user's chakra, the higher their level of kai release will be and as a result the higher level of genjutsu they will be able to break.
Using Tsukuyomi against the likes of SM Naruto or Hashirama would be a completely different ball game because their chakra is superior to the CS chakra Sasuke used to break it.