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You can deploy and AMP page, and set a meta header that announces it exists. This is not an explicit opting in to Google's AMP cache though. That's what the author means by implicit license.

You opt in to the internet by making content available on it too, so are we rolling back to the free copying of last millennium?

As discussed in the article, some content management systems (like hosted WordPress.com instances) produce AMP variants of each published page. The author/rightsholder may not be aware of this and thus has neither expressly or implicitly agreed to anything of the sort.

Then the author should pay attention.

If wordpress by default said on the footer "content licensed under CC", how would it be my fault if I reuse the content, just because the author didn't bother to change that?

Do you have to opt-in to AMP e-mail? Keep in mind that the sender then holds the copyright. For that matter, can your web hosting platform opt-in to AMP without your express consent?