From Giphy's end they can still so useful traffic analysis by generating a unique URL for the search result obtained by User A and subsequently sent to his chat cohort. Each cohort app retrieving the same URL could then be enumerated. It's not user-identifying but it would generate a sort of contact graph.

Also the Signal service could do a transparent TLS MitM between the app and Giphy instead of a passthrough TLS tunnel and the user apps would be unaware. In fact from that page I'm not sure they're even doing a tunnel anyway.