https://git.sphere.ly/Lloir/android_kernel_htc_evitareul/tre...
The way manufacturers "mitigate" baseband to main CPU compromise is by using a protocol that allows no initiation from the peripheral device (baseband). It can only talk to the main CPU via a serial-like protocol, not access its memory directly.
Other routes, such as side channel leakage or possible flaws in the main CPU software that interpret messages received from the baseband are still a potential source of problems, but there is no such DMA capability in the HSIC protocol.
It is valid to have general distrust and annoyance with the spy agencies for their actions to create backdoors. But there is no technical basis for this article's claims, and the author should retract them.