WB are a big part of the problem but to not lay any blame at the port house is also not correct, especially if they're not owned by the publisher, like this case. They are often bidding and claiming they can do XYZ in amount of time for amount of money. So at the very least they need to be responsible enough for themselves to know what they can do. It's not their first rodeo. They released Borderlands 2 Vita, one of the more notable bad ports around. If they can't manage the work, they should not be agreeing to contracts stating they can.
Of course, there's a lot of factors we don't know about like if WB or Rocksteady were late with stuff and whatnot, but IG is not blameless.