The point that Fastmail made in the article, though, is that if Fastmail is handling the encryption, then they know your key. So Fastmail cannot protect you against Fastmail's servers being compromised[0].

And the article linked from their article (hilarious itself) pointed out that this basically comes down to State Actors (Mossad as the example) or Everyone Else. If it's Mossad then there's nothing anyone else can do to protect you (because they can do secret-squirrel things through special security courts and stuff like installing black boxes on undersea cable routers). If Everyone Else, then the normal run-of-the-mill transport encryption is probably good enough, just don't click on any herbal boner-pill adverts.

tldr; If you want to be protected, you have to do PGP at the local machine level yourself.

[0] Given that Fastmail are Aussies, and that our idiotic government have given the spooks full, secret, access to anything they want, then this is a very real prospect for anyone that five-eyes might be interested in.