This is because WhatsApp uses Apple's emoji set, which essentially uses SoftBank mapping for backward compatibility in a Private Use Area (PUA), long before emoji was standardized in the Unicode.
The character "" (U+E409) is in the Basic Multilingual Plane's (U+E000 - U+F8FF) PUA. Since it is in the PUA, there is no standard definition for the character. While the character is supposedly rendered as , SoftBank (and Apple subsequently) apparently uses the PUA for their emoji, and U+E409 "coincidentally" renders 😝.
Some other characters in the PUA that are rendered as emoji on WhatsApp (U+E400 - U+E40F):
Note that this is entirely WhatsApp's doing. This also affects WhatsApp Web and also WhatsApp on PC. However, other apps/OSes may render the character differently or otherwise, fails to render.