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.