Clarify the use of external clock source with RP2040

[regicidalplutophage]

The datasheet states:

If the user already has an accurate clock source then it is possible to drive an external clock directly into XIN (aka XI), and disable the oscillator circuit. In this mode XIN can be driven at up to 50MHz.

However it is not clear what is meant by "disabling" the oscillator circuit. Does it mean disabling it in XOSC CTRL register? Looking at the Arduino Nano RP2040 connect, that doesn't seem to be the case. Please clarify.