It's more nuanced than "it looks better in his opinion".
Typewriters used monospaced fonts. The old consensus on the "two spaces" rule came about because it made things easier to read on typewriters. Source code editors are also monospaced, so why not the same rule for the same reason?
It also allowed him to more easily modify the text in his editor. The problem with this blog post is people are taking it to mean "two spaces are superior", when he is really saying, "two spaces are better for me, with my tools".
Typewriters used monospaced fonts. The old consensus on the "two spaces" rule came about because it made things easier to read on typewriters. Source code editors are also monospaced, so why not the same rule for the same reason?