As I suspected that article is referencing devices that have the powerline devices built into the mesh units so they are mesh and powerline all in one.
In your case (if you wanted to) you can still use stand alone powerline adapters but it looks like you would have to use the deco in access point mode.
Have you seen the article on access point vs router mode on the deco’s?
https://www.tp-link.com/uk/support/faq/2399/
Where you connect the Sonos and nas will have more to do with what mode you choose, in your case if it was me I’d use the access point mode to avoid the chance of running two routers and the networking issues that can cause.
If the nas was connected to the virgin router and Sonos to the deco router for instance you could well see connectivity issues as they would be different networks (segments).
In ap mode you should be able to disconnect one port that’s in use connect the deco and use the second port on it for the device you disconnected. Saves buying a switch for one port.
All good fun eh!