Same here. I just read SftD (and the rest of Ender’s Saga) for the first time after bouncing off it when I was younger. I loved Ender’s game and The Shadow series but never made it any deeper into Ender’s Saga as a kid. As an adult, I greatly enjoyed all of the followup books to Ender’s game and the whole concept of “speaking for the dead”.

Thought so as well

If an activist speaks up about a group of people being marginalized, they're speaking 'for' those people. 'About' would be if it was descriptive speaking (perhaps an anthropologist's article) but if it's in support of those people than 'for' is the right word.

BTW the object doesn't need to be people, it could also be animals or the environment or even abstract ideas like liberty or justice.

That‘s wrong. Speaking for someone means from their point of view, and it ususally implies a moral permission to do so.

Speak for yourself.