This is not unusual for exchanges but I still strongly prefer River over any other service. The Bitcoin has settled but the fiat transaction has not settled. It may show as such on your end but on the banking side this can take about a week or more. There are a lot of things that happen that can slow down the settlement process so they are quoting 20 days because they know that for sure the transaction should absolutely settle or reject by that time. During this time if the transaction is rejected by the banks and is clawed back you end up in a situation where a bad actor can steal Bitcoin from an exchange. Once it is withdrawn there is no possible way to recover it. Exchange writes it off as a loss and often the fraud isn't worth perusing an individual bad actor over.

Strike has determined this is an acceptable risk and they most likely have active monitoring solutions in place to help keep that risk low, but this is risky for other reasons and no easy task to monitor.