It depends entirely on your threat model. Their claims about privacy are fine if what you're worried about is ads and tracking rather than government surveillance.
Fastmail isn't end-to-end encrypted, so there is no requirement for a backdoor applicable to Fastmail. The Australian law is completely irrelevant to Fastmail except for FUD comments online.
The Australian law allows the government to order telecommunications providers to assist in intercepting telecommunications and assist in implementing the technical capability to intercept communications (aka backdooring stuff).
Practically for email there is no need or reason to back door anything to do this though.
If asked they’d just pull the content straight from the server, same as Google/MS assuredly do for the US government. So I don’t think the threat model or risk of back doors is increased.
