In order to validate transactions, Arbitum uses multi-round proofs in an interactive verification game between the prover and sequencer, whereas Optimism previously used single-round fraud proofs executed on L1. This was until a flagged vulnerability led to the disabling of their proofs. Optimism still operates today without fraud proofs meaning users must trust the Sequencer node (run by Optimism PBC). This isn’t currently of too much concern due to the centralised nature of both L2s (one must also trust the Arbitrum Sequencer node).
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It is not true that you have to trust the Arbitrum sequencer. The only role of the sequencer is to establish an ordering of transactions. The sequencer cannot forge a transaction and cannot change the outcome of a transaction.
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