What IS the difference? I don't really get how something can be 'semi-legit'....surely, just by defintion, that makes it non-legit?
Semi-legit shiny Pokemon are Pokemon who are legit shiny, however the Pokemon themselves are cheated/the items used to get them are cheated.
For example, Arceus can be SRed for if you hack the item used to unlock the in-game event. So it can be a semi-legit shiny. Or you could be using the random encounter code to find a Jirachi and end up finding a Shiny Jirachi.
But yes, by definition it's not legit. But opinion plays a huge role in whether or not it's a real shiny, some people think that semi-legit shinies shouldn't be counted. I think, in the case of Arceus it's okay. But in the case of Jirachi it's not. I feel like I am rambling.