ehhh not necessarily. It's not a vulnerability in and of itself, but a scan of a block of ips with his in it, or a full scan of only his ip will let the scanner know the ip has equipment operating on it. It would throw a flag, so-to-speak. All in all, still nothing terrible, but not optimal.
If a port does not have any exploitable services running on it, it doesn't matter. If you were to do a port scan of any ip address which reports known ports closed you can not run an exploit against it. So, closed is as good as stealth.
The way it works is like this. Assume two scenarios. The first is that his ip is not a target of any person, organization or group. During block scanning, if a closed port is found, his ip now gets added to a list of "possible" ips to hack. The simplest example is that routers have known and easily exploitable backdoors built in, but there are others.
The other scenario is that if his ip is a target of any person, organization or group, if they find a closed port, they know that the ip is valid and there is hardware operating on it, at the very least.
It's mainly theory, but this stuff takes place all the time too. Even kids that don't know the first thing, run scripts that someone built on an IRC channel and they are owning machines and building botnets thousands of machines deep, running channel servers, placing ftp servers to put what they want to serve on machines, and all types of stuff.
If that closed port wasn't found, those script kiddies wouldn't go to step 2 for his ip.
Even with all that though, that Gibson site meant a lot more than it does now when people did not have routers and connected straight to their isp's modem. And also before XP started having a firewall. I think that was SP2 or something like that. And especially before Microsoft made changes to network shares ("net share", "net use" commands) and disallowing root access when a null (blank) password was set for any given account.
Gibson himself used to have troubles with kids even, running these things built by "real" hackers because he was a huge target because of who he is. A 13 year old kid that didn't know much would give him grief often - lol.