Sliding wasn’t working by selecting the edges and adjusting in the operator panel.
As a workaround, I was still able to use the panel by selecting the edges in Edge Mode instead, activating edge slide, then tweaking the value there. No need to slide a little first, just press GG, mouse click, then tweak the value in the operator panel.
They may want to add to the video. I remade the model and selecting faces + GG by itself still always resulted in a vertex slide rather than an edge slide. Obviously something has changed or it’s enough of a problem that they could add a small explainer like they do in other vids. Hopefully they do that soon, since I would imagine all future versions of Blender will operate the same and this is now out of date and a bit confusing based on the other posts I’ve seen.
Thanks for the real solution. 
Welcome to this site.
This bit of the course annoys me, it is a stupid way to raise the base. Mikey just uses it for some reason, I think as he had recently introduced sliding verts. He even says it is not the obvious way, or some such like that.
Has often caused people problems.
Just go to wireframe front view, box select all the base verts and G grab; Z to constrain direction; 1 for distance; click/enter. Done.
