If it's a simple proxy tunneling HTTPS traffic to Google, Apple probably doesn't know anything about the content of the queries, and Google doesn't know who sent them. If each kept records, they could get together and combine them to get the hashed URLs, but still a much better situation than directly querying a single endpoint.

Signal actually uses a similar approach to anonymize queries to GIPHY from users of its app. https://signal.org/blog/giphy-experiment/