spi_transceive_cb is not asynchronous in nucleo_h753zi

[2ahunter]

I am using the spi_transceive_cb function on an nucleo_h753zi board. I want to take an incoming ethernet message and split it into multiple SPI busses that operate in parallel. The documentation states that this function is asynchronous and that the function will return immediately, with this caveat: "Note that asynchronous API calls can still be blocking if the bus is already busy. The functions will block until the bus is available to start the requested transfer."
However, I find that the function doesn't return until the transaction is complete. I can toggle a GPIO just prior and just after the call to spi_transceive_cb and see that it's blocking, moreover there aren't any delays prior to calling the function or after the transaction is completed. How is this asynchronous? Am I missing something?