Is it impossible? It is hard to prove impossible. But this is not what I would expect backdoors to look like.
In fact this makes me think that the information about how these were generated must have been lost very early in the process, or over-compartmentalized away from the wrong people. It seems like the NSA should have been positively delighted to show that these were the origins of the seeds as they are decent "nothing up my sleeve" numbers: https://en.wikipedia.org/wiki/Nothing-up-my-sleeve_number Advertising that fact should have been advantageous to the NSA if that is indeed how they were generated, so my best guess is that the parts of the NSA that should have been showing us that must not have themselves known.