A marketing agency has developed its own risk assessment approach as part of the ISMS...

ISO/IEC 27001 does not mandate the use of a specific risk assessment methodology. Organizations are free to choose their own approach as long as it is systematic, consistent, and capable of producing valid and comparable results. This allows organizations, such as the marketing agency in the question, to adapt the methodology to suit their specific needs and business context, provided it complies with the requirements set out in the standard.