i think they mean that signal on desktop does not encrypt their content at rest, which is acknowledged and not an issue they are intending on addressing.
But it seems to have recently changed? I’m learning thus as I wanted to find a source.
Source: https://candid.technology/signal-encryption-key-flaw-desktop-app-fixed/
Have used my BT headset for almost a year and never had an issue. What are you referring to?