The Go language is by itself more than enough to get a trademark. They only additional thing you can do with a font is get a design patent... for the font.

It's more likely that they don't feel the need to use the font they gave you (for your code editor) on code samples on their own web pages. As it's a custom font, the text would be susceptible to pop-in, or would visibly switch from one font to another, both of which are ugly.

Their CSS says this:

    #blog .Article[data-slug='/blog/go-fonts'] code {
      font-family: 'Go Mono', monospace;
    }
    #blog pre,
    #blog code {
      font-family: monospace;
    }

So they have no specific font preference for code samples, they only show their Go-specific font on the blog post about it. They prefer fast loading and no jank over custom font.