Ducter It's listed in the steps for flashing Google factory images which he mentioned he was trying to see if Google's web flashing recognized his phone.
That's odd. I've flashed back to PixelOS several times, and never needed to enable USB debugging, nor was it ever mentioned on flash.android.com. But maybe Google's tool does suggest that, but only for users of macOS? If so, it's always good to follow the official instructions.
In any case, USB debugging is not needed to flash GrapheneOS. The install issue isn't there...