First time was with someone behind the official GrapheneOS twitter account, and second time was with Daniel Micay, both times about the hardware supposedly preventing the ability to update the firmware even when running alternative software.

In reality, the only thing that prevents the firmware from being upgraded is the default software not implementing such features (because PureOS does not and will not distribute non-free software or firmware). A user who wants to do that can reflash everything without modifying the hardware in any way or even opening the case. Both WiFi flash and the M4 core-based solution described in https://puri.sm/posts/librem5-solving-the-first-fsf-ryf-hurd... can be bypassed completely to load the firmware from rootfs just by using a different kernel and bootloader, and disabling SPI flash read-only protection is a single line of code change in the device tree.