Once I got over my fear of clicking their links, which I assumed would open a new page (but instead just expanded a pane in-line), I really enjoyed it. I’m very wary of opening new pages. (Also, I first tried to hold-click on the link to open in new tab, but it just behaved like regular text and highlighted, which led to a momentary confusion. I would have preferred a more obvious indication of what would happen when clicking, like a down chevron or something.)

I also assumed those were going to be links, but after a second of confusion I really liked the side pane with animations. It adds a lot to the article and it's more pleasant than the usual alternatives (lightbox on top of the text, or opening a bunch of tabs).

Off the top of my head, I'm not sure how else you'd visually communicate "this bit is interactive on click/hover but isn't a link." Maybe a different text color (without underline), background color, outline (replaced by the colored highlight bar on hover), or a slightly larger and more distinct icon to replace the generic 'image' icon?

Yeah, styling it as a link makes it a bit unclear as to what it's going to do.