Bitcoin is Byzantine resilient because of PoW and Game theory. Bitcoin follows Nakamoto consensus, but all Byzantine consensus algos are only resistant up to 51% or less.

The bigger question is how likely a 51% attack or sybil attack is within bitcoin and under what conditions can we make it less likely.

With PoW there is at least physical limitations and better signals that limit sybil attacks vs PoS. Nothing is trustless or completely immutable but we can get closer to these ideals with decentralization and the right security mechanisms.