> The security implication here is that the telephonic operating system has access to the same system resources as the application operating system at the same time, right?

It shouldn't. As per https://grapheneos.org/faq#baseband-isolation:

  > The baseband is isolated on all of the officially supported devices. Memory access is partitioned by the IOMMU and limited to internal memory and memory shared by the driver implementations. The baseband on the officially supported devices with a Qualcomm SoC implements Wi-Fi and Bluetooth as internal sandboxed processes rather than having a separate baseband for those like earlier devices.