He explains that by using a line he can more easily move it "when the quarters move".

Using a border would require removing the border and adding a new one every three months. A line can be dragged around at will. This also makes it more useful for miscellaneous illustrative purposes.

I'm an excel noob but doesn't table mode fix all this.

Possibly because a border is part of a column so if you insert a new column adjacent to a border you have to remove the old one and redraw it which is pretty annoying because its tucked behind a few clicks and menus.

A line however floats relative to the columns so you don't have to remove/redraw it. At worst you might have to move it.

Visually both are good enough.

Moving a line is 1x mouse click and a wrist flick,

Borders are more work than that.