I think people who want private OSes and people who want offline storage for things have a lot of overlap

The point they are trying to make is that an ubersecure phone with GrapheneOS will have a certain target demographic.

Among that target demographic SD card slots have a demand.

I can't help it if people have not yet realized the huge cost of memory is a major factor in phone price increases, and that SD cards are a great way to have your full media library on your phone without being beholden to "the cloud".