The most logical answer is that one of the defendants provided their password to decrypt all the emails (for the quotes mentioned in the news article).
It’s also not uncommon for law enforcement to spread misinformation about such services in order to discourage usage.
Hi there, the allegations made by Mr. Ortis are entirely false. A link to our refutation of these claims is available here: Tuta Is An Independent Company And Not Linked To Five Eyes Secret Services .
We are currently working with our legal team to take action against these false claims and will be able to provide updates when more information is available.
Most welcome to the forum @Tuta_Official! Glad to have you on here.
Thanks 
@Tuta_Official can you comment following articles: one, two?
I think their official statement covers enough of the allegations.