There's no mention of any attempt to simply find the people that originally made it and asking them how it was made, since protection was halted in 1989 most of them should still be alive.
Surely that would have been more cost efficient than reverse engineering the production process, but of course the government would have had little incentive to save money.
So just reading between the lines this is less of a story of how the USA "forgot" something, and more of a story of how some sprawling bureaucracy couldn't bring itself to pick up the phone, presumably due to some combination of information siloing, the complexities of re-granting some retirees security clearances etc.
My reading of the reference to notes is that they were following some written material left behind by the original team, not that these were notes acquired from ex-members of the team in 2000.
It's not at all beyond the realm of possibility that the people who developed the process or were intricately familiar with it were still around.