People use terms without even knowing their meanings.
Rivalry means competing for same purpose.
In what Universe, Zoro and sanji are competing for same goal and thus are rivals?
Some people just spout nonsense without actually understanding the context and meaning.
It's a gag rivalry and that's why oda portrays them competing for gag stuffs like " who will hunt bigger dinosaur".
Heck, sanji doesn't even has primary role of fighter to be called rival of zoro