I don't think any hotel staff would be dumb enough to 'guarantee' an early check-in for free as early as discussed here. I think it is smart policy in COEX's case to quote a 30%-50% rate for early check-in.

On the other hand, I've arrived early (unannounced) dozens of time to various hotels (including IC COEX) and they are able to accommodate to the best of their availability. In the case of COEX, I arrived ~6am, and they took my luggage and apologized that they don't have any clean rooms available, and told me to come back after 9am. I did and got my upgraded room. I'm only a lowly PLAT and it happened ~5 years ago.