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/
You only mention your laptop is running out of space so you need to get a new computer? does your laptop have a soldered SSD? If that’s not the case, I think the reflex should first be to see what storage you can get your laptop so that you can keep using it rather than discarding it :(