The RM are part of the Royal Navy. Therefore, why shouldn’t they have the same rank insignia as the rest of the RN? A bit like the RAF Regiment are a part of the RAF and have the same (mostly) Rank structure as the rest of the RAF.
And it’s not a “plot”. It’s in the OS version of the RN Command Plan (pg40), which is helpfully found on the front page of the RN MoDNet.