I fail to see how it's not agile? Roadmaps aren't against agile. If marketing is low priority and there's a large backlog of high priority work, marketing related work is never going to be considered when planning a sprint.
The only way around this is to have more dev capacity or to prioritize marketing work.
We outsource a lot of the marketing tech and website related work so marketing is not blocked by our own development team. We don’t see marketing tech and our Wordpress website as our “secret sauce”. The devs are just focused on product development.