Right, that’s what I understood. So using a VPN, a CSS will be able to identify that my phone is active, but not the content I’m accessing, or who I am accessing it from, correct?

From my understanding your statement seems correct, but it’s also lacking a bit. Unless you also randomize your mac address (grapheneOS does this) they can still map your position and visiting times. Additionally not all of your phones data goes through the VPN, something like a phone call/SMS isn’t encrypted unless you’re using an app to make the call.

The previous comment said VPNs do nothing against this type of attack- were they just referring to identifying your device?

Yes, they are thinking of a VPN as a privacy tool, not strictly as a security tool as in your example. Privacy will be compromised.

CSS wouldn’t be used to spy on your network traffic; if they wanted your internet data, they’d have much simpler methods to collect it than CSS (and they wouldn’t be able to decode most of that data anyways in normal cases).

or who I am accessing it from

What do you mean by that?

Suggesting that a VPN could mitigate stuff relating to CSS is like wearing a floating vest 24/7 when flying in a Boeing plane: you might feel a bit safer with it on, but it’d probably be smarter to have a parachute instead.