A very interesting and quite well written account of how homomorphic encryption might be used to match, as max of inner products, two embedded vectors corresponding to query and document.

What still bothers me, or I failed to understand, is that the server still knows it returned the document "How to dispose of a body", even though it has no idea what my query was.

That doesn't seem helpful and still leaves the only solution to private search being anonymity. The search engine never needs to know who made the query unless it intends to sell them something or spy on them.