I would call that system secure. It does not just rely on an obscure password but is actually restricted by a list of whitelisted networks.
The failure in that case is only that the admin didn't consider that normal work might be done from home at some point or that the middle or upper manager thinks that he should be able to freely administrate his critical infrastructure from anywhere...
IP whitelists break so often for "unanticipated reasons" that I've lost all sympathy for not anticipating it. Doubly so for using a whitelist to lock yourself out of the whitelist admin.
It's so common the security community should make it a meme to spread awareness: Don't get pwned by DHCP while running from SSH 0-day RCEs.
The failure in that case is only that the admin didn't consider that normal work might be done from home at some point or that the middle or upper manager thinks that he should be able to freely administrate his critical infrastructure from anywhere...