The author seems to be under the impression that that set of glyphs not in the ASCII set is somehow special. That is not he case for any reasonable modern OS. Your font rendering engine does not care whether its painting a or a F or a . As long as it knows how to draw the glyph it will, and if it doesn't it will make one up.
If your browser could not draw the tub and pantheon above chances are neither would black-screen. The author seems to be an accomplished coder, but I can't see how the magic of JavaScript will make teaching your font engine how to paint a glyph go away.
It's not the font-rendering he's taking exception to. His argument seems to be that UI elements like the chevrons and icons in vim-powerline shouldn't be rendered using fonts at all. They currently are because traditionally terminals only have text-rendering to work with, but it's clear he'd rather have that UI be HTML instead.
If your browser could not draw the tub and pantheon above chances are neither would black-screen. The author seems to be an accomplished coder, but I can't see how the magic of JavaScript will make teaching your font engine how to paint a glyph go away.