Hello,
To get access to the QSPI Flash, you need to install the patch as described in the first message of this thread (Patch).
If you’re not feeling comfortable with git, just copy the 6 files of the patch one by one. Pay attention to the fact that updating the package through the board manager will erase these modifications.
IMHO, the mbed implementation is not ready/fully functional, if you have the choice, use the “not mbed” one.