That's not true. Look at their FAQ [1]:

> GrapheneOS is permissively licensed and is usable for building devices with an immutable root of trust. GPLv3 is deliberately incompatible with these kinds of locked down devices, unlike GPLv2 code such as the Linux kernel. This means GrapheneOS can't include GPLv3 code without forbidding use cases we want to support. GPLv3 is no problem for our own usage, but we don't want to forbid using GrapheneOS as a replacement for the Android Open Source Project in locked down devices.

They're not against GPLv3 and, but they explain why it's not suited for a project like GrapheneOS.

[1] https://grapheneos.org/faq#:~:text=In%20some%20cases,locked%...