Not true. You have a lot of games developed in these recent years that still use this flow. Black Desert, Arena Breakout, other free games where you usually need to login first before the game launches, usually they check the game for updates outside of the client. So, no, Metin2 is not the only app doing it. Talking about this feature, i actually don't see a benefit, its just different. If you want to make your metin2 different, yes, but this does not make it better. Actually, the autopatcher is a faster flow then this one. The only benefit of this, is if you are able to bypass the autopatcher. In that case, the ingame check would force you to close the client and update. Other than that i don't see a benefit.
Anyway, good thing, its the first time i see this implemented.