That’s the way I originally interpreted it as well. However, I don’t think it’s completely correct.
First, I interpret it as no one using the Free OnShape subscription would be permitted to use any Public Documents for commercial purposes.
However, since you’re effectively granting rights to anyone to “use, copy, modify, merge, publish, distribute, sublicense, and/or sell copies of the Document,” it’s entirely possible for someone (including yourself) to export the document and use another tool that would allow for commercial use. The specific details of how this could be enforced would be open to litigation.