I had to use a Windows box as my Macbook Air M1 didn't even recognise the thing.
As I rebooted with
adb reboot bootloader, it rebooted into the Onyx logo (which I presume is the bootloader). Other ADB commands like opening a shell on the device are recognised.
However, when I type the Fastboot commands on the bootloader screen the only prompt I get is
Code:
< waiting for any device >
which indicates that the device is not being seen in the bootloader mode.
It disconnects and reconnects on its own every 15 seconds or so. It's shown on Windows's Device Manager with QUSB_BULK:SN[SerialNumber].
In order to leave the fastboot mode I held the power button for some time, until the tiny blue LED light embedded on the power button flashed a couple of times, and then it went back to the system. I've been able to repeat this, so it should be safe if you want to try something more.