You probably know this but technically a forest doesn't have to have trees to be termed 'a forest'. From wiki:
Although a forest is usually defined by the presence of trees, under many definitions an area completely lacking trees may still be considered a forest if it grew trees in the past, will grow trees in the future, or was legally designated as a forest regardless of vegetation type
I was also lead to believe the following:
The term
forest in the ordinary modern understanding refers to an area of wooded land; however, the original medieval sense was closer to the modern idea of a "preserve" – i.e. land legally set aside for specific purposes such as royal hunting – with less emphasis on its composition
Looking at the map, The Forest doesn't actually mark the summit, rather and area of the eastern flank. Presumably the fell top has been associated with this as many usually are. Presumably the area was either forested, will be forested in the future or was associated with hunting