You might be able to disguise it as debugging/development code that was mistakenly left in there. And instead of a hardcoded list of targets it could pull down the values in a more creative way. But at the end of the day that probably wouldn't stop a talented reverse engineer from figuring out what was going on.
Android apps can also contain native code. Indeed, WhatsApp includes such libraries, to help with Curve25519 encryption, video encoding, voice over IP, and other functionality.
But it should be straightforward enough to see if text messages or UI elements (suppress key change notification) are being change depending on the output of those libraries.
Even with perfect e2e encryption protocol added, what's preventing WhatsApp developers (FB) from adding in a feature of the app:
if local.user is "TargetUser007" { takeDeviceSnap(); sendDeviceSnapshotToFBOverSameEncryption(); }
Wouldn't this not be ever verifiable unless you ARE that specific user and it's too late?