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You didn't understand.

Obviously it's a requirement to not be able to predict the future of a prng given output from one. However (and the video discussed this!) that requirement may be violated, either accidentally or on purpose.

Therefore, these protocols should be implemented in such a way that they are more robust to defects in prngs. One such way is, obviously, not putting the output of a prng in the clear, then using the next bits out of that prng.