If Signal and WhatsApp allowed users to choose own software to encrypt data and submit encrypted blobs to the Signal and WhatsApp servers, to use those servers as temporary storage, and thus to enable users to use any protocol they choose, including own protocols, then those apps might be comparable to something like "CipherGist"

Instead those apps require that the Signal Messenger, LLC and Meta Platforms, Inc. companies do the encryption, according to their rules, following their protocols^1

The user cannot easily switch from one company to another, the Signal client cannot be used to send/receive messages via WhatsApp servers or vice versa, the protocols are proprietary. Approaches like "CipherGist" are more generic and work with any server that allows upload, temporary storage and download of data

If want to avoid any server storing blobs then can use the server only to provide port numbers for establishing peer-to-peer connections

1. Perhaps Signal and WhatsApp users send empty messages with encrypted blobs as attachments