I don't think they consider systems like San Jose VTA, San Diego, Calgary, Edmonton, Sacramento, Portland and other places running modern light rail to be "metros." Mind you, if that's the case, I don't think they should include Muni, even though their light rail vehicles do run in a subway for a good stretch.

There isn't IMO a real bright-light definition between "light rail/streetcar" and "heavy rail/metro/tube" transit systems. Extreme cases are easy. It's the places where the one merges with the other that are difficult.