Crystal Extended support releases are ultimately a harm reduction measure. They're meant to exist to assist people in migrating to a supported device as soon as possible while still getting some updates.
The unfortunate misconception is that people think that because their device is receiving GrapheneOS updates, it's perfectly secure to use. GrapheneOS doesn't want to mislead its users, so it's important for the project to reiterate that people should migrate to a supported device with every opportunity.