Yes exactly. Before I got my current Pixel 7a I tried to flash a used Pixel 6a (Verizon variant) and ran into the problems above.
Without an unlocked bootloader the install process will fail at step two, Enabling OEM unlocking, of the install guide at https://grapheneos.org/install/web.
The OEM unlocking setting in developer options will be grayed out on all Verizon devices, even if you paid full price for an unlocked phone (not on a payment plan - some OEMs also lock phones while under contract so you can’t steal the phone and stop paying. Verizon just locks them all regardless).
There are numerous websites with dodgy software that claim they can bypass the restriction, but in my experience 100% of them are scams. Sometimes well meaning redditors will tell you they got theirs working, but they’re conflating OEM unlocking and carrier unlocking (which, so I’ve read, Verizon employees are specifically trained to conflate)
JFC nuclear weapons are horrifying