Yes and no. The browser is making an HTTPS request to the AMP Cache for the document. The document's integrity is ensured by the cryptographic signature of the publisher's origin which the browser verifies. As far as the integrity of the document, this is the same conceptually as the fact that your wifi router is actually serving the document to your browser. The AMP Cache cannot modify the document, and the publisher may choose what to sign or not sign.

Entering the URL directly will make an HTTPS request to the URL's origin server, as it always has. There will be no AMP Cache involved in that request, but it will work just the same.