Conclusion

Agile and DevOps are not substitutes.

DevOps leverages agile’s Definition of Done within the CI/CD framework.

While ideally DevOps engagements should follow agile methods, even Waterfall or Iterative models can leverage DevOps by integrating Dev-Test-Ops. Hence, it is not appropriate to think that agile is contained within DevOps or is a sub-set of DevOps. Both stand on their own, independently.

To conclude, agile and DevOps co-exist and complement each other.