I think they meant, “don’t have internal and guest WiFi connections use the same subnet”. If you’re giving non-employees passwords and access to a network that contains sensitive resources that’s a big security red flag.

Many companies also use IP whitelisting for external services/systems, so if someone from outside the company joins an internal network they now have access to them as well.

Edit: Actually, they point out those exact reasons if you expand that entry in the list.