C8 MCT oil is really to feed the brain, it is used for biohacking - improving one's performance, and there is wealth of anecdotal data from places like Silicone Valley that it works.
Keto diet is used to direct our bodies' metabolism to not rely on glucose, but to switch to using ketones.
There is a small overlap: some people find that when they come out of keto (diet failure, diet mistake, or similar) they feel awful. So using ketones like MCT oil, or coconut oil, the logic goes, stops a keto flu symptoms. Not sure if it is really effective?