It’s not, unless they’re some sort of cryptography expert with a peer-reviewed white paper pending publication. The Signal protocol (GPLv3) is extremely robust and has almost no capacity for metadata generation, and both the app and server-side code are under the AGPLv3 (technically if they were compromised they could use different, unaudited server-side code, but refer back to “basically no metadata”). Signal has essentially no capacity to be compromised; they can’t even bait and switch users with a pre-compiled app whose source code isn’t the publicly available one and actually has a backdoor because their builds are reproducible and it would be caught immediately.

Maybe they take issue with the crypto bullshit, which is valid but doesn’t compromise messaging security. Maybe they don’t like that they took away SMS, which I completely agree with, but also actually makes it marginally more secure. Either way, I seriously doubt if they had any mathematical insight into Signal being “compromised” that they would be here skulking around on Lemmy right now.