Could not find much info about that claim, but context probably was that data is not possible to be accessed without compromising device, e.g., not possible to get info over network or by compromising some central location on remote server because there is none and all that data is stored locally.
Microsoft was claiming that the data would be inaccessible to hackers (which is not true).
Signal claimed the exact opposite: that once it’s on your computer, messages can be seen by malicious programs on your computer.
Signal was caught having less than ideal security. Microsoft was caught lying.
Could not find much info about that claim, but context probably was that data is not possible to be accessed without compromising device, e.g., not possible to get info over network or by compromising some central location on remote server because there is none and all that data is stored locally.