Scenario: In 1925 an agent is deployed to Russia, infiltrates an intelligence agency, and does something cryptologically noteworthy. It is mentioned in this 'top secret umbra' document. In 1955, said agent, while still acting covertly in Russia, makes further cryptologic "contributions" to Russian intelligence agencies. In 1975 the ciphers from 1955 are still in use, and the USA intercepts information and identifies Soviet spies. In 1995, the USA feeds misleading information to those formerly Soviet spies. In 2016, the USA still cares about the misinformation provided in 1995 to spies identified in 1975 thanks to ciphers from 1955 which were broken by a long-dead agent who started work in 1925.

Seems plausible enough to me.

No the agent that did something 90 years ago, was still there and did something 60 years ago. Which influenced something 30 years ago, which led to something 5 years ago and is still in use.

It's a stretch, but there could be a chain of events that they want to remain hidden. They don't want to hide what happened 90 years ago, they want to hide who did it and what else he may have done in the future or had an effect on.

Yep one time pads are still in use and those where invented way before ww1