Supposition without any proof/evidence. With third party tools or even windows task manager you can terminate the game at any time and this doesn't dupe items. You can discard and transfer items between bags right up to the end of the timer. What you say would only make sense if all inventory interactions were blocked while logging out outside, and they're not. Even if we accepted this as true though, it STILL doesn't need to be 30 seconds in a safe location. It doesn't take 30 seconds for server checks to be performed on inventory operations, it happens within a single send and recieve. They could reduce it to as little as 3 seconds even if you actually had internal knowledge of how the game works and could prove what you claim was true. The game doesn't need more than the amount of time it requires to perform actions like transferring between bags, selling, or throwing away. Thus, at worst case, they could set the timer to 3-5 seconds instead of 30 in town areas.The reason for this is potential duping.
But no- the real reason for the timer is so you can't abuse logging out for a combat advantage / avoiding death. I don't think the logout timer and inability to perform use or crafting actions in the mog house is connected atl all.