I heard that the only way Cardano will solve contention issues is through centralization

Centralization is indeed one way to solve the challenge of contention in a UTXO-based blockchain like Cardano, but it is not the only way to solve this. SundaeSwap Labs wrote an article about some of the ways that DEXs and other dApps can solve contention issues, and will implement its own unique solution in the SundaeSwap DEX.