I think Jens is correct, the notice is related to font redistribution not use. If you are redistributing the font (either loose or embedded in a document) you might want to include the license as a text file with the related files (PDF/fonts).
Not to mention the dollars Google sunk into the Noto fonts. Also Chinese and Japanese fonts are expensive, and I'd wager that a lot of money is being spent in that space. Latin likely has the volume, but there's definitely money in non-Latin.