It doesn’t sound too far off from what I’ve seen. The details of the sprints may not be worked out, but a dev team might have several months of high priority project queued up and marketing might be deemed lower priority. So getting extra work done from another team can sometimes take a very long time.
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.