If I put a file with certain content into Google Drive, then Google Drive, or a subpoena for my Google Drive data, will return exactly those contents.

Hence, it is reasonable to apply any distinction to the content as a user of Google Drive sees it, and not as it may be stored on the backend. Hence, if the data WhatsApp pushes to the Google Drive API is unencrypted (and we're talking about the data, not about the HTTPS-encapsulated form that passes over the network), it is reasonable to call it "in clear text", and it wouldn't be reasonable to call it encrypted.