I read it a little differently.The ruler of the 10th house cusp in 11th may suggest that the highest priority for the native,tends to be bound together with larger groups/friendships and associations.
The friendships that one makes mean the world to the person or there could be a cause /organization that one joins that encompasses the core of their purpose and where they want to make social impact.
With Neptune squaring the Sun in the 11th, it could suggest that there could be some unrealistic expectations the person has with regards to the friendships/organizations they become a part of.
With Neptune,there could be disappointment suffered and some disillusionment that may come about in relation to the 11th house themes.
Perhaps one changes their feelings about the organization and friends they are a part of or perhaps the organization/friendships "deceive" the native in some way.
With the Sun ruling the 10th and in the 11th, much of the "wipe out" that can happen with Neptune can end up having you question not only your own purpose of living(MC) but who you are at your very core(Sun).
An example of this is if you join a church and within this establishment,you find your purpose(Mc).
All the people around you know you as a devout member of the church(Sun) and much of your personality is groomed within this establishment.
But now say you discover the church you serve in has some under the carpet,seedy dealing with drug lords and child pornography(I know I know, I am reaching. But just stay with me on this as this is the Neptune square at work)
So now you are in an identity crises (Neptune-Sun square) because the church that you thought and believed was so "holier than thou" turns out to be not so clean.
Furthermore, you now question whether the purpose(Mc) that you have always associated with being a part of the establishment, is worth continuing with?
That is Sun in 11th ruling the 10th square Neptune loose example.