I fail to see why this is bad advice. Sure you could just disable the firewall on your computer on a local network. But that’s under the assumption that you can trust everything on your local network. What if it’s a laptop? Do you also trust any public networks you may connect to on the go? Having firewall both on the router and on your computer provides an additional layer of security, and I think that’s good advice in general. You can for example set it up to only allow incoming connections when connected to your home network for example.
To be fair I haven’t configured a firewall either on my laptop. But that’s out of lazyness, not out of good practice. Good practice would be to have a firewall enabled. Just because something is unlikely to happen statistically doesn’t mean it’s bad practice to take steps to protect against it.