In certain unusual circumstances, I think IO psychologists were required to get licensed in the past so it may be a holdover from those days.
There are tons of IO psychologists out there though, so not knowing a function of your particular experiences.
I think it is on there for the same reason that a lot of the material is on there. It is a test of reasonable foundations of broad-based knowledge that defines "psychologist" which makes sense given its a growing and fairly popular subfield. Some areas of IO are actually extremely relevant to certain aspects of clinical practice (work-life balance, burnout, training), though I can't say I remember whether those were on the exam. Makes about as much sense to me to include as some of the more nuanced social and cognitive stuff. I can't say disentangling the 7-8 different theories of identity development with roughly 95% overlap that I think I memorized has much relevance to my clinical practice either...