I've looked through the documentation and I'm not seeing a way to import an XML policy that was generated on the primary DSM manager into a tenant account via the API. Does this mean it would need to be coded by hand? Our policy file is ~6000 lines and this is not an ideal solution.
Alternately, is there a way I'm missing for cloning a policy into the tenant account through the API?
I think it should be very hard. The very first thing I would say is that rules (IDs) could be very different from primary DSM. Even by API, these ID are still different.
When you export a policy and related objects to XML, you can then import them frpm XML again through the console: https://success.trendmicro.com/solution/1113917-exporting-and-importing-deep-security-manager-dsm-policies
For API-based export and import, you would have to write some scripts to save data and then create/assign the appropriate objects on the other end. Note that these assignments would have to be based on something other than the API ID, since as HsiehYY's answer points out, the IDs of rules and other policy objects may differ between DSMs.
P.S. I work for Trend Micro R&D
