I'm not sure why tptacek specifically warns against using Dropbox. My guess (and I emphasize that this is just a guess) is that you can't rely on Dropbox (or Google Drive or Microsoft OneDrive) to keep your data out of the hands of a state-level adversary. However, encrypting your data before putting it into Dropbox should address that concern. Is there something I'm missing? Is it that cloud folders like Dropbox make it too easy to accidentally share information in cleartext?
If you have a device that's relatively well hardened against attack, why subvert those protections by giving a copy of your secrets to a third party who isn't (and can't be, from a legal standpoint) as well protected?
Why give a copy of your secrets to an adversary that's 10 to 20 years ahead of the rest of the world, crypto-analytically speaking?