The only thing I want to push back on is that internet activism isn't helpful. It's incredibly important for education, because most people don't see or hear anything but CNN or Fox in their daily lives, so Facebook and Twitter have become an excellent opportunity and tool to get important causes in from of peoples' eyes who would otherwise never encounter them.

In the context of the blog post, I found the background information about the Chagos refugees incredibly educational, it was just a terrible call-to-action. Like you said, the CTA should have been to donate or to volunteer, or to spread the blog post in order to educate others. It became counterproductive when it became about a highly-specific, questionably-impactful action that no one reading the article can likely affect.

Every significant organisation, government, big company probably had done something terrible at some point.

Yup, which is why it's basically impossible to be an ethical consumer these days. "The Good Place" did a really entertaining exploration of this idea.