It may have been me being confusing, but it wasn't the AO3 team who said that about multifandom content. That was my own interpretation about why those specific posts got reported. Unfortunately as they are now blocked I can't point to them as an example. But those four were the only posts I've made (of hundreds) that included (a) brief commentary and (b) were multichaptered (20+ chapters) works with a different canon being discussed in each chapter.

So it wouldn't be the multifandom aspect of a post, I think it's a long, chaptered work with just brief paragraphs in each chapter that will get the wrong kind of attention.