I'm still working my way through the videos....
This has probably been covered before but had anyone else ever heard them use this phrase before? Not me... But it's a clever and as per usual misleading choice of words.
If they are indeed Guardians of Doctrine, they are very poor ones. If so JWs would believe the same thing now that they did in 1879 and we all know that's not true. Also the word 'doctrine' is misleading. It is not so much JW doctrine that changes. It is INTERPRETATION that changes to match their plans and intention.
In a way the Commission was caught out by this phrase and claim. As it has never appeared before to my knowledge in any literature or correspondence, and certainly is not in common JW usage, the Commission couldn't prepare and disect that claim, showing it to be untrue and yet another lie.