Regardless of what we suppose were their motivations regarding the suit, I think that there is an argument to be used for using the more precise/universally understood terms "thread" and "public post" in the ACP.
I still get users wanting to send "PMs." XF is awesome software, but some ideas will turn out to be good and some ideas will not be as good. One idea that I don't think turned out so well was to create new terms like discussion, conversations and messages instead of existing terms like threads, PMs, and posts that most forum posters already know.
The sense that I get is that my audience tends to be less computer savvy than most, but most people still find our site using Google, so there are a lot of people that are savvy enough to know the cnventional terms like PM. As a result, different people use different terms on my forum and the newcomers get confused. For example, a less computer savvy person will use the term "conversation," because that is what the software calls it, whereas a more computer savvy person will use the term "PM," because that is the standard term. Newcomers have no clue that they are talking about the same thing and get confused, which makes my job harder. XenForo's design is, like Apple's, very "Xen" and effortless. However, even Apple screws up sometimes.