Yeah, it's stupid advice. Even if you isolate the untrustworthy ISP router with firewalling and VLANs and a device you control and trust, who's to say that hostile device does not have a microphone, and is not transmitting information about who's at home and their movements via emf analysis?
And who knows what else we are not even thinking about.
Keep hostile/untrustworthy hardware off your premises. Best bet.