While you do not have to use MettleCI to use S2PX there are some benefits to using the two in conjunction with one another:

Unit Tests

You can use MettleCI Workbench or MettleCI CLI to automatically generate unit tests for your legacy Server Jobs. When you convert those Jobs to Parallel equivalents S2PX will also convert your Unit Tests (including test data) to suite the newly-generated Parallel equivalents. Running these new tests will provide irrefutable evidence that your Parallel Jobs produce the same outputs as their Server counterparts (insofar as is demonstrated by your chosen test data).

DevOps

One your S2PX-generated jobs are tested you can deploy them downstream by checking them into a Git repository which triggers a MettleCI-enabled build and deployment pipeline.