I was surprised by the news. In the press release, they present Dagster as the "outcome layer" and Prefect as the "execution layer", but I never really thought of them as complementary in that way. They do have different orchestration models but Dagster already has its own execution layer (ops, graphs, jobs, executors) that its asset model is built on.
Maybe they're planning to gradually separate "outcome" from "execution" and make Prefect the runtime engine but for now there seems to be quite a bit of overlap.