Yeah, that's a dubious claim as well. Here is how GNOME Software presents a sandboxed application:
https://i.imgur.com/VMsj6mk.png
Is it a bad interface? Sure. Is the list of permissions hard to find? Hell yes. Is the Sandboxed emblem kind of misleading? Yeah. But here (in at least GNOME Software 3.36) you see a list of the relevant permissions available to the application. Nobody is lying here.
For contrast, here is an application with a stricter sandbox:
https://i.imgur.com/lCGgA1B.png
The point here is both of these are sandboxed. Each sandbox just has particular holes in it. For GNU Octave, more than seems appropriate. Users misunderstand that in the same way people misunderstand "autopilot" and that's a good issue to bring up, but then I don't think non-technical users understand what "sandboxed" means anyway and technical users should be able to take one more gorramn step and learn that "sandboxed" obviously comes with particular limitations [at least in the present day] or it isn't going to do anything.