Google Fonts does see the page you are on, in the `Referer` header. According to mitmproxy:<p><pre><code>    host:              fonts.googleapis.com
    Connection:        keep-alive
    Proxy-Connection:  keep-alive
    Accept:            text&#x2F;css,*&#x2F;*;q=0.1
    User-Agent:        Mozilla&#x2F;5.0 (iPhone; CPU iPhone OS 9_3_1 like Mac OS X) AppleWebKit&#x2F;601.1.46 (KHTML, like Gecko) Version&#x2F;9.0 Mobile&#x2F;13E238 Safari&#x2F;601.1
    Accept-Language:   en-us
    Referer:           https:&#x2F;&#x2F;blog.filippo.io&#x2F;self-host-analytics&#x2F;
    Accept-Encoding:   gzip, deflate
</code></pre>
(I agree with the rest of your comment; the best thing to do is host the fonts yourself.)