The confusion might come from the fact that Dagobah is often described as a swamp planet or swamp world. This doesn't mean it's a system, it just emphasizes the dominant feature of the planet's surface.
In the Star Wars universe, a system refers to a collection of celestial bodies orbiting a star, including planets, moons, asteroids, etc. Dagobah, on the other hand, is a single celestial body orbiting a star.
Therefore, the term "system" is not used to describe Dagobah in any official Star Wars material.
