It's not a graphics mode. It is just a resolution from the graphics mode, repurposed for the text mode. The reason possibly being the 640x480 resolution is universally supported while 720x400 is not - for example my HDMI TV can still display 640x480.
CRTs were still commonplace though based on the bios dates in the blog. So 720x400 would still have been well supported. It’s strange that they did this!