You are correct, but there is a difference between being sim unlocked and bootloader unlocked.
All Verizon 4g phones are sim unlocked, meaning it will work with a non-Verizon sim out of the box, provided the phone has the right radio bands.
An unlocked bootloader is necessary to run custom software, kernels, and makes rooting much easier.
All phones come with the bootloader locked for security reasons. However, Google allows the user to unlock the bootloader if they wish.
Conversely, the Verizon bootloader will have a bootloader that will not be able to be unlocked. It's theoretically possible that someone could unlock the bootloader through a security exploit or hack, but I doubt it, especially since anyone that really cares about modding and rooting will just buy from Google.
It'll be much more difficult or impossible to root the Verizon version or run custom software since the bootloader is locked.