Of course that asks the question of why use the graphics mode for text, which I would suspect was simply to show the seemingly obligatory Energy Star logo, but these Intel boards don’t have it.
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!
This apparently was a text mode not a graphics mode. It would have been a tweaked text mode since the standard mode 3 text mode that you can set via int 10h is 720x400.
The energy star logo was also displayed in text modes - by using custom font glyphs!