Yes, you're missing something about ASCII, and it is that the piece you're commenting on is not ASCII art, it's ANSI. That means the artist has 16 foreground colors to choose from, and 8 background colors. He can only use "raster blocks" to vary the amount of the foreground color against the background color, which is a very difficult technique, in my opinion.
Pixel art is mainly done with a mouse and a color palette with numeric values, and ANSI is done by typing and mixing colors in a more instinctive way. That would be the main difference.
Sorry for the late comment I was browsing through ANSI Daily Deviations and found this. I hope I didn't sound annoying. Have a nice day