You should not take wiki as iron rules. This is one of the more unclear tags. If you do take it strictly, it only includes " green space" and "pitches, nets ". It is mostly associated with entire  grass fields, often containing multiple courts. The cover photo is simply a `=playground` that can be enclosed in it casually, if not excluded. 
They have different surfaces. 5k `=recreation_ground` has `surface=` to differentiate. 