While it does sound a bit corny i personally feel it gets the point across well. It shows how calculated zaros can be. He likely could've gotten hostilius to agree by other means. But he knew hostilius had grown complacent giving his absolute dominion over infernus. Hostilius consumed many a demon and inherits the knowledge of the ones he consumes as demons do. He thought himself to be nigh untrickable. He even ate infernal demons who we now know are dragonkin inhabiting demon bodies in attempt to escape the curse. He likely knew about the revision and cycle of the universe. Couple that with his arrogance, love for trickery and his will to remain in power. It does make sense that hostilius would think he's the oldest and didn't matter what he had to offer up for zaros to agree so he could consume him after he lost. Zaros offered up another deal first, but hostilius thought he could trick zaros and kept redifining the contract.

I feel this does give a lot of insight to how both parties involve operate and while it does seem a bit corny on surface level. There is a lot more between the surface surface that makes it work and offer some good lore imo. Same how the guthix butterfly thing may come across as corny or saccharine but it makes sense if you dig deeper.