That's not major in terms of the overall value, but they could still be one of it's biggest investors, the other 95% could be made up of other investors who are valued at much less. That could also be spread across multiple entities and different investment vehicles within the group.
Unless you had access to the full list of investments they manage and the list of investors for each one there's actually no way of knowing how much PIF has invested or who the biggest investor is. 5% doesn't sound like a lot, but 5% of 70bn is 3.5bn, which could actually make them one of their biggest individual investors.