Incorrect logic for corn on the config page

A patch updating the logic if refresh_once is true set to 1 otherwise use the corn configuration.

The logic is correct. The refresh_once is used for specific services that only publish once a day new exchange rates.

So you can at the plugin level specify (force) that it's only running once a day.

An example is ECB plugin \Drupal\commerce_exchanger\Plugin\Commerce\ExchangerProvider\EuropeanCentralBankExchanger

which in definition have set this property to TRUE

/**
 * Provides EuropeanCentralBank.
 *
 * @CommerceExchangerProvider(
 *   id = "ecb",
 *   label = "European Central Bank",
 *   display_label = "European Central Bank",
 *   base_currency = "EUR",
 *   refresh_once = TRUE,
 * )
 */

So for ECB, you can't run import more than once a day.

By default this value is FALSE - which means if the plugin does not specify it, you are allowed to set on the configuration page how often cron runs are needed