that's what the y-buffer is that the article mentions in the front-to-back rendering section.

it tracks how tall each columns write is so you can use it to only write the diff between it and the voxel behind it, skipping writing anything at all if the voxel behind is shorter than the current height.

So once you're done rendering front-to-back, you've got a y-buffer of highest-writes you can slap your blue sky across from highest-to-screentop on each line, avoiding the need to clear by write the sky to the full screen before starting the render.