You get the alpha of your combined layers including the text and shape layers. I'm a bit confused... What do you expect to occur?
That tutorial is teaching you how to make a track matte. The goal is to make the fabric image only visible where the animated large type is.
The channel menu is a preview tool. It allows you to preview the color channels and alpha channel of your composition. it doesn't preview layer by layer but instad if the composite result of all the layers visible in the comp.
The alpha channel preview isn't targeting anything. The areas of the comp that are opaque display as white, the ares that are fully transparent display as black and any semi–transparent area displays as gray.
The black background color you see on the left of your comp is the comp background, it's just a preview, it's really empty space.