Well, it can mean a lot of things. AFAIK most current smarpthones have an application CPU where the "normal" OS lives (Sailfish OS/Android/iOS/etc.) and a baseband with its own CPU that runs some totally proprietary RTOS no one really knows much about and which handles all the communication with the cellular network (calling, SMS, data, etc.). These these CPUs generally communicate with some sort of IPC - for example if the user uses the phone app to start a call, the application CPU tells the baseband CPU to initiate it. How tightly coupled these two CPUs are can make quite a difference, as you can generally tell what the application CPU is running while the baseband CPU is basically running an unauditable binary blob sitting on an always online wireless connection. IIRC in some cases it even starts first during boot and/or can manipulate content of the main memory without the application CPU knowing about it. Fun and games! For that reason the Neo 900 project went the way of having a separate baseband module that communicates with the application CPU via clearly defined and easily to audit channel & which can be robustly disabled (by turning it's power supply off). So it could be the Librem Phone projects wants to do the same thing as Neo 900 (fully separate baseband module). Or they just rephrase the status quo (individual application/baseband CPUs with unclear separation) in a positive way - and I would not discount that given that the whole project seems to be a bit marketing heavy and fact light.
Unlike some other smartphones do, Neo900 won't share system RAM with the modem and system CPU will always have full control over the microphone signal sent to the modem. You can think of it as a USB dongle connected to the PC, with you in full control over the drivers, with a virtual LED to show any modem activity