> This allegedly violates the GPLv3 clause that requires the release of the source code for the entire work if they are combined to form a larger program in or on a storage or distribution medium, such as the flash memory of the STM32F407 micro-controller on the board, and more recently in the update images Prusa supplies

This makes 0 sense as presented here. The GPL explicitly distinguishes mere aggregation from combining in a way that makes it a larger program. Putting both the proprietary bootloader and the GPL OS on the same disk / in the same system makes 0 difference to whether the bootloader is covered by the GPL or not.

If this were true, then any phone, any PS5, any Linux laptop, and almost all consumer hardware would be in violation of the GPL (note that the wording quoted there is also part of the GPLv2, it's not GPLv3 specific).