The server has a snapshot of Wikipedia, sitting in memory. The server is blind to which article is requested because it computes a homomorphic dot product between an encrypted one-hot vector (encrypted under a key that only the client knows) and the total set of articles in plaintext. The encrypted query does not reveal anything about the requested index (ie it is semantically secure).
The 'magic' of homomorphic encryption is that the server is able to take an encrypted one-hot vector of bits, and a vector of plaintext articles, compute a homomorphic dot product, and produce a single ciphertext encoding the single desired plaintext article. This single output ciphertext is crucially also encrypted under the client's secret key, so it reveals nothing to the server.
The 'magic' of homomorphic encryption is that the server is able to take an encrypted one-hot vector of bits, and a vector of plaintext articles, compute a homomorphic dot product, and produce a single ciphertext encoding the single desired plaintext article. This single output ciphertext is crucially also encrypted under the client's secret key, so it reveals nothing to the server.