I bet 100% of those secret maps are flat.
I bet 100% of those maps have a notation in Russian stating the projection used to render that flat map of a spheroidal earth.
I bet 0% of the cartographers assumed the Earth itself was flat even while drawing the projections onto flat paper. The giveaway: they're accurate.
Coincidence?
Convenience.
I think not.
Yeah... we know. You keep demonstrating this.