How odd that the text doesn’t mention the solo option.
Any layers tagged with the solo option will show up in previews and all the others will be hidden and stay that way even in rendering until you deactivate all solo-ed layers (I’ve made that mistake too often).
I edit my tutorials directly in AE, and solo just the video and audio layers for easier previewing. You can click drag to turn all off / on easily enough (and I bet there’s a shortcut key that I don’t know about)