[Client isolation] confines and restricts clients connected to the Wi-Fi network. They can’t interact with devices connected to the more secure wired network, nor can they communicate with each other. They can only access the Internet.
Indeed, this is either AP isolation, or an IPS-like function in the router. My cheap Sitecom router has a function labeled 'Block NMap scan' and it does about the same thing, depending on what scanmode i use.
However this is not done by the ISP :)
With NMap, if you're scanning the devices there, seeing just the gateway, you interrogate the LAN (local area). The ISP will usually not have a clou this is happening.
If you were scanning external devices, say, a website, this would go over WAN (wide area), this is something the ISP could see and possibly influence.