It's not decentralised. If it was, it wouldn't have a maintainer. It's centralised in that every X months (I don't remember, 3 or 6) the chain just increases difficulty without bound, grinding to a halt if the latest consensus rules update hasn't been installed that pushes back the deadline another X months. This is how they make sure everyone is using the consensus rules that Vitalik currently prefers.

The difficulty bomb was a mechanism to ensure eventual migration from Proof-of-Work to Proof-of-Stake, which happened in 2022. PoS no longer needs a notion of difficulty.