Legion and Skynet are two separate AIs, so while I understand that the 101 that killed John Connor no longer had to obey it's mission after Skynet was erased from future existence, I don't see how one of Skynet's products could have any info on Legion's plans.
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I can't find the exact quote, but the T-800 mentions that it can detect "temporal displacements" which let him calculate the exact time and location a Terminator would appear. Then Sarah interrupts him and says "That explains the 'how' but not the 'why'." That's pretty much all the movie does to explain it.
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3Importantly, those temporal displacements appeared in advance of the time traveler's actual arrival - he could see the wake before the boat arrived, so to speak.– T.J.L.Nov 8, 2019 at 13:10
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