I mean 
& 
are mentioning about Demacia and Noxus. So they're from Runeterra. Its ok until you think of it:
Where the hell are 
and 
? I mean Darius/Garen were never "final boss" material. More like sub-boss lieutenants and all the sudden Garen is God King of Demacia and of whole runeterra.
Super Galaxy Rumble mentions Bandle City.... Does that mean that its a "Pseudo Canon" skin as well?
With that logical, the Marvel superheroes are real because they mention real world cities like New York City.
No, they're not semi-canon. Just alternate universe of Runeterra where Garen & Darius are leaders of their respective nations. The nations themselves are much different than their canon counterparts.
The fuck does semi-canon even mean...
It's either canon or it's not.
These skins might be alternate-reality (of fiction) but they are clearly not canon.
