https://grapheneos.org/faq#future-devices

Hardware, firmware and software specific to devices like drivers play a huge role in the overall security of a device. The goal of the project is not to slightly improve some aspects of insecure devices and supporting a broad set of devices would be directly counter to the values of the project. A lot of the low-level work also ends up being fairly tied to the hardware.

TLDR - Google open bootloader, able to relock the bootloader with custom keys, AOSP, hardware binaries freely accessible, as well as the hardware security features on pixel phones.

Selected Items

• Support for using alternate operating systems including full hardware security functionality

• Complete monthly Android Security Bulletin patches without any regular delays longer than a week

• Vendor code updated to new monthly, quarterly and yearly releases of AOSP within several months to provide new security improvements (Pixels receive these in the month they're released)

• Isolated radios (cellular, Wi-Fi, Bluetooth, NFC, etc.), GPU, SSD, media encode / decode, image processor and other components

....

Is google pixel the only group capable of making a secure phone? No, anyone could do it, but only google pixel IS doing it.