There is a difference between security and privacy or freedom of use. Locking down the device to only allow a subset of apps that Google has some control over (by requiring developers to register) is a measure that can increase security, even though it obviously takes some control away from the end-user.

The fact that the play store is not exactly known for exceptionally high standards w.r.t. malware, or that there are lots of valid concerns that come along with a company controlling who is allowed to supply apps for the device is a different topic.

Don't mix security and freedom. They're commonly opposed to each other.

That's not at all what Google announced.

It has nothing to do with devices. It has to do with OSes, most notably OSes certified by Google, which GrapheneOS isn't.

Also, it will be possible to bypass it even on certified OSes.