The first two ISP books both include a paragraph very similar to this one in Interstellar Expeditions in their introductions:
Some aspects of those books have proven to be completely accurate, others shade the truth in different ways, and still others are completely bogus. I wouldn't treat anything in Interstellar Expeditions as canon information for any official purpose. But that's what makes it fun!
I understood that, it is fun as plot book for role-playing. Unfortunately, if someone needs solid information, about periphery, ISP3 isn't it. Why i always termed these books as "the books of lies" because you can't use them unless if just for optional adventure. Especially you want to have hard-facts on a factions or what is known. We won't have anything really outside what was published beyond FM:3085 and older sources. Whatever was mentioned in FM:3145 and Era Report 3145, is are only tidbits of information of near Periphery worlds in small references. Nothing huge, like ISP.
This is my own option, so forgive me if my perception on this subject is vague. Before ISP series, we could always rely on the books to deliver us unshakable information. yes there some stuff in them is grany, but Jihad Hot Spots, least it was new reports are considered canon in public knowledge, ISP delivers quasi-canon or semi-canon.
Only reliable information someone going to get on the Wastes, is properly from Star League era books, such as Field Reports, Historical: Liberation of Terra books, Field Manual: SLDF. Which have some information on previous territorial states including the Outworlds Alliance's expanded holdings.
Unless, ISP3 has some sections that note this is facts, this is where you can branch out to make your stories on.