Yes, it must be something like that. Similar to ersatz coffee where I believe the Nazis resorted to using acorns instead of coffee beans. https://en.m.wikipedia.org/wiki/Coffee_substitute

This article talks about the problems of sourcing rubber as a raw material and the solutions - https://historycrumbs.com/retreading-tires-the-armys-ingenious-solution-to-rubber-shortages/