That involves "source routing" though (from what I get, that means controlling the route a packet will take, at least partially). I only see one ACK being returned, not multiple, so apparently the attacker is able to receive the SYN+ACK here. This means it's a different attack.
Then again, I did read (parts of) the RFC from '81, some paper (from Morris iirc) about predicting the ISN from '85, lots of research in the 90's and early 00's regarding predictions and preventing it... but nothing actually just guessed the number, and people seemed to agree that "if the ISN is unpredictable then yeah it's secure right?" That's why I considered this a new thing.