If you allow users to write and upload their own BPMN as well, then yes - it is a generic workflow as a service.
Both predefined workflows where users can supply user-defined code for steps, and the inverse (pre-defined handlers and user-defined workflows) fit the allowed definition of:
selecting them from a restricted set of pre-defined programmatic services made available by the provider of the Commercial Workflow Service).
Making the predefined handler a generic execution engine that executes the content of user-supplied content - whether through a custom header, or an uploaded file, still means both the workflow and the handler are user-defined, which is a Commercial Workflow Service.
Basically, you need to be providing some business-domain-specific logic that is the value in your offering (either in the BPMN or in the handlers) - rather than just supply a generic workflow service where all the domain-specific logic is supplied by the users, and you are charging for the workflow orchestration functionality as a service.