If a NAT made a device unreachable from the Internet, it'd be more secure then a device that's reachable.
You could theoretically leave management ports unsecured etc, because there is no danger - they can't be reached after all.
It just turned out that techniques such as NAT hole punching were developed, which made this rationale invalid. Because your devices could still become reachable.
Yes, this specific technique requires a an active part on both ends, but this active part can be something completely innocent, such as an activeX applet in Internet explorer 6.
Or a Freeware they installed, and now opens up the whole network, not just the device is installed on.
This is all from the very beginnings of the Internet, things weren't as explored back then, and that's why NATs were considered a security feature back then.
Modern malware is obviously much more advanced so we intuitively know why it was never really providing any safety. Hackers were mostly doing things for bragging rights, cryptolockers weren't a thing, the current default malware setup of having command hosts that compromised hosts check for new commands to execute weren't even thought of. In these times, NAT looked like a security feature, because it kinda looks like it.
Please explain why the ability to establish an outgoing connection through NAT, when you already have a sidechannel, is a security issue?