To publish to the main 16+ pool I need to do "an age check"? But it seemingly doesn't matter what the result of that check is? I'm confused.

The age check is part of Roblox as a whole. It dictates what games you can access and the range of ages you can communicate with. They also extend age check to who you can develop games with, as the Roblox Studio IDE is basically a real-time multiplayer 3D environment with chat.

Accounts without age verification can only play all-ages games, chat is fully disabled, and they cannot publish any content onto the platform.