Thanks for enlighting me; I always use without hyphen
adb reboot bootloader
which is in line with syntax, but for curiosity tried couple of moments ago on Moto G4 with hyphen successfully
But, in what mode you issued command
Because you are using Windows platform, acquire appropriate drivers for that device.
In developer options, both Enable OEM unlock && USB debugging must be enabled.
Regarding your problem, I briefly looked for same or similar, excluding Samsung and found
LGV20, Wayteq xTab100qcr and Glass.
I'm quite interested in this issue, though never worked with Sony, so specifics of their devices are unknown to me.
Try with volume- or volume+ button combinations for entering fastboot mode.
If that doesn't help, look at this.
I remembered I actually had contact with Sony device - it had some sort of custom FRP - my Xperia Theft Protection (MXTP) - but haven't worked with it. I planned to use Xperia FlashTool.
From Xperia FlashTool you should be able to reboot to bootloader.
Please, do inform what you have done.