Australian law allows the government to demand access to your email, and Fastmail hold the encryption keys. That's not the case for Switzerland.

What encryption keys? Fastmail doesnt offer encrypted email

Data at rest in encrypted volumes. That's not saying much, obviously; you'd hope everyone is using encrypted storage for user data by now.

Not GP, but I would imagine they’re referring to Australia’s not-so-stellar track record with encryption lately.

It's Swiss. From memory they have pretty good legal protections for privacy, at least compared to Australia.