Article doesn't mention there are strong and weak ETags, only presenting the strong variant (those hashes in double-quoted strings).

Weak ETags are rare(r) beasts that look like `ETag: W/"foobar"` and mean that while response may be not byte-to-byte identical (e.g. it's a dynamically generated JSON document), it's still semantically the same - which can be useful for APIs (if caches implement all the logic correctly, of course).