"Points" as used by Photoshop basically refers to pixels. That is, points==pixels.
DPI stands for "dots per inch." This matters for printing, but doesn't make any difference to the image data and thus it doesn't make any difference to the image as displayed on a computer. Also, dpi can be changed later without affecting the image (although you have to be a little careful with Photoshop, because it will try to alter the image; you have to manually tell it not to.)
From glancing at the tutorial, he draws big. For example, the top image is at 25% (I would guess it's really around 3000 pixels wide,) and that's just one panel of the comic. This makes sense; the rule of thumb when doing art, on a computer or otherwise, is work big. It is easier to work in detail that way. Also, with a computer image it's trivial to shrink down later, whereas going bigger means killing the resolution.
Ultimately, the vocabulary surrounding image dimensions and resolution are pretty muddy. For example, the difference between screen and print resolution, or the relationship between image size and pixel dimensions, can be weird. I've spent entire class sessions explaining this stuff to people who were confused.