Basing a historical novel on supposedly found long-lost or long kept secret :facts: which are in actuality just part of the fictional story is a common writers’ tool.
I have enjoyed some very well-written “what-if” stories.
“Are you certain the found documents were legitimate?
This is the first I have heard of such German plans.”
https://en.wikipedia.org/wiki/Imperial_German_plans_for_the_invasion_of_the_United_States
The plans were very real.