A sandbox provides a highly controlled environment. I'm not entirely sure what you mean by serial port or how it would work but the Librem 5 product description mentions nothing this, nothing of a baseband sandbox, nor any sort of controlled environment. They simply state that the baseband will be separate from the SoC. This design is of course better than most modern smartphones which integrate the baseband in the SoC but it is by no means a sandbox. If you have 5 minutes spare, here is a video which provides a quick overview of what the Neo900 guys are doing.