As I understand the author, the implication is that the baseband firmware is closed source. So, it's possible that Google/Apple have done an audit. But, in my experience, and the author's suggestion that's unlikely to have happened.
I belive he is saying that the baseband code is closed source even to google and apple. Thats where much of he danager lies, it has exterordinary levels of access to tne device and there is no accountability for its security.
I think the author is radically oversimplifying the access the baseband has, based on the erroneous notion that the baseband in a phone design is connected in the same fashion as a peripheral is in a PC design --- hence the "IOMMU" bit.
