Here are two possibilities.

1. The mining pool that included the replacement transaction didn't have the first transaction in its mempool and accepted the second transaction when they received it.
2. The mining pool that included the replacement transaction has enabled full RBF.

Any node that has enabled full RBF treat any transaction as RBF-enabled and accept the replacement transaction even if the original transaction has not been flagged as RBF.


And it may worth mentioning that RBF isn't a consensus rule. RBF is a policy and miners are free to include any valid transaction they want.
A miner can include a transaction paying zero fee even if there's an unconfirmed non-RBF transaction broadcasted earlier with a very high fee spending the same coin.