Could this a debugging instruction that was mistakenly left enabled in production or possibly even an intentional backdoor?

Most modern ISAs like RISC-V provide no way of directly accessing physical memory regardless of privilege (you have to either disable paging or setup page tables to point to the physical memory you want), so it seems unlikely that one could accidentally implement one.

In case of an intentional backdoor it seems surprising that it would not be authenticated with a secret key, but maybe they are very incompetent.