I just choked on my food reading this nonsense, then realized the article came from a popular far-right blog that is also known to spread pro-Russia disinformation.

The dEeP StATe y’all

Anyway. If you are reading this and wonder where you can get more context, there’s a Forbes article explaining it: https://www.forbes.com/sites/thomasbrewster/2021/02/08/can-the-fbi-can-hack-into-private-signal-messages-on-a-locked-iphone-evidence-indicates-yes

TL;DR: it’s not a vulnerability in Signal that allows it, it’s through iOS vulnerabilities. In this case, to bypass the lock screen and extract content of a phone they physically possess.

It is also possible to do that remotely. Again, through iOS vulnerabilities (either 0day or known bug in unpatched iPhone) turned into an exploit chain that bypasses mitigations, gains root privileges, and drops an implant that extracts content.

Again, this has nothing to do with Signal. It’s the nature of mobile exploitation and the reason why keeping your phone up-to-date is recommended.