It is true. NAT only changes the source address used for outbound connections, it doesn't deny inbound ones.

You don't need to take that on faith either -- you can just test it.

Go ahead, make an inbound connection to my dev laptop. I'll even give you the IP address: it's 192.168.8.21.

Get me onto the network that's on the WAN interface of your router, disable the firewall on it, and I will.

How do you want to go about doing this? Although, 100% of the time people have asked me to do this they chicken out at actually doing it, so I suppose you will too. You might prefer to test with some network namespaces instead.