Not just an implication, isn’t this explicitly stated? Any superpowers in a cold war are constantly spying on each other and bending whatever rules they think they can get away with.
And they rightly assume their enemies are doing the same.
You can also get your own modem, but that doesn’t change the fact that yes, you need to use a VPN for any external traffic you don’t want your ISP seeing.
However, ISP controlled routers provide them direct visibility into your LAN, which they otherwise wouldn’t see. This gives WAY more detailed information about you and your behavior, i.e. the original article.
PSA: If you can afford your own a retail router, it’s always a better idea than using the one provided by your ISP. Make no mistake, Xfinity is absolutely collecting (and probably selling) data on the traffic of every device on your network.
That’s not at all what this means. You’ve always been free to add any additional parental controls to your kid’s devices, and that hasn’t changed.