Keeping internal IP addresses and hostnames secret?

Corporate network, security, problem professional of nearly 40 years, and I've got into a little professional argument with someone:

Is it important to keep internal IP addresses and hostnames secret? It seems like "security through obscurity" to me, and you can't do anything with those IP addresses and hostnames unless you already have access (when you could just look them up in DNS anyway). Plus, most companies uses the reserved private IP address space anyway (and NAT). (Yes, of course, …

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