Reductio ad absurdum: if serving fonts from Google is “unnecessary” and leaks information, so would be using any CDN service to deliver any content.
Wrong. In this case it's illegal because Google Fonts can be served from the origin server as they are downloadable. The same does not apply to sideside CDNs. However, I believe NPM CDNs are the same scenario, so I'd start serving packages from the server as well.
I guess this particular case seems somewhat reasonable, but where is the line.
Also note that Google Fonts is a lot more that just hosting a download. It has different font files for different browsers for max compatibility as well as font splitting so that you aren't downloading too many glyphs and weights that you don't need for this page. Reimplementing Google Fonts isn't trivial.