I assume that they were themselves convinced that it would happen, I've been told that Lenin only signed the treaty of Brest-Litovsk since he didn't think it would matter anyway as it was only a matter of time before the world-wide revolution that would reshape borders would come. Yet it seems a little presumptious for them to base their plans for the future on this. How likely was this to actually happen? Were other nations afraid that it would?
At the time of the Russian Revolution, Russian communists were pinning a lot of their hopes on the belief that the German revolution would succeed. With one of the most developed countries succumbing to a socialist revolution, the basis would exist for a joint Russo-German partnership to build socialist society.
It's speculation on the likelihood of this leading to a global socialist revolution, but the belief by Russian communists was tangible. The German revolution ultimately failed which left Bolshevik Russia isolated politically.