What exactly does this mean? Doesn't the baseband processor exclusively handle the mobile communications? I thought the parent comment was more to do with OS running on the main processor.
Formerly, yes. But from 4G phones on, they rather communicate via DMA, i.e. the baseband part of the SoC has full access to the device's memory.
Also see the relevant section in the Wikipedia article: https://en.wikipedia.org/w/index.php?title=Baseband_processo...