Actually, an "Active Cover" is something different: it allows interaction with the app from the application switcher.
Which is, imho, a novel thing.

What you mean is a "Cover" that is updated in realtime.
According to the linked document, applications are explicitly allowed to perform actions like computations etc. in the background. Which, imho, is exactly what multitasking is about.
The link i mentioned just gives a little guidance on how to improve background activity of apps.
It even does not forbid that you still perform fancy 3D rendering or HD video playback or other fancy stuff and update the "Cover" with this while in background.
It just gives the "hint" that apps shouldn't do it.

Edit:
To correct myself: at one point it says that, "All animations must be paused and, if possible, unused resources released.".
Which kinda would prohibit doing fancy stuff like 3D rendering or video playback.
Otoh, I think we can only truly explore the capabilities of Sailfishos once it is officially out.