Feudalism's relationships were claimed to be mutually beneficial.

They did have benefits, and those benefits seem to fit in nicely with the model you presented. What the exploiter (lord, baron, earl, king, etc) gives to the subject is protection from other exploiters who may be worse or who at very least will be 'more'. Even enforcing laws would just be modelled here as preventing exploitive relationships. If Y is exploited by X then X will benefit from killing potential exploiter Z. This is a 'mutual benefit' in the X-Y relationship but it does not suggest 'mutualistic', in the defined sense.