Take a look at Snowden, Poitras, and Greenwald's communications and the recording of the footage for Citizenfour as an example here. They had all already stopp

Take a look at Snowden, Poitras, and Greenwald's communications and the recording of the footage for Citizenfour as an example here. They had all already stopped relying on any closed system to keep their communications secure. They were all using Tails to communicate through Tor without leaving a minimal unencrypted footprint. At the point where you are relying on Tor to communicate and are explicitly using Tails to keep your communication secret then why would you start relying on a Macbook, running a proprietary OS, with non-open crypto, to hold your secrets if you're already scared the neo-Stazi are going to break down the door and steal your unlocked, decrypted Linux machine that you have all of the trust in the world in? For that reason alone you would skip the Mac.