This advice makes sense given the threat model. However, it might not make sense for someone in Edward Snowden's role. If I were a military agency with a big budget, I would backdoor the shit out of every phone, enforce cultures of secrecy inside companies like google, apple, facebook, intel, qualcomm, at&t, and off any executive that interfered with the mission. Then I would pay experts to spend their lives on internet forums asserting that devices with two cameras, two microphones, wifi that can function as radar, an unremovable battery, a closed-source operating system and root access only available to a major US corporation via ssh, are the most secure computing platforms in the universe. That's just me though, if I had a lot of money and lust for world domination, neither of which I possess :)
Edit: removed sentence "Most mobile devices have baseband chips with DMA"
No, iPhone basebands do not have direct memory access. This is a myth that will not die. The baseband on an iPhone (and on modern Android phones) is connected via a serial bus, as a peripheral. Both Google's and Apple's security teams consider the baseband an adversarial device. This has been true for many years, just as for many years people have been popping onto message boards to confidently inform us that basebands have direct access to memory.
Edit: removed sentence "Most mobile devices have baseband chips with DMA"