It's really hard not use variable fonts now (IBM Plex is not variable). They just make so much more sense and reduce the package size sometimes significantly.
Presumably they mean proportional-width rather than monospaced.
Instead of having to download and serve a separate font for each style (bold, light, etc.) it's just one font that interpolates those attributes resulting in much smaller font sizes. Here's a list from google fonts just to see some examples... https://fonts.google.com/?vfonly=true