I think it's kind of misleading since it's only done for compatibility of the produced binaries. CentOS 5, until recently, was the actively maintained distro with the oldest kernel, libc etc. versions. So if it runs on CentOS 5, it is guaranteed to run on anything. This is different from companies still using CentOS 5 and are running active services on it because they simply refused to upgrade. Python or rather PyPi does this too. They aren't using CentOS 5 Despite of it's age, but because of it.

Is this to produce binaries on the oldest possible distro so they'll run anywhere (symbol versioning etc.)?

That being said, CentOS 5 hasn't received security updates for year or so (or 2?), so maybe there's a security risk in continuing to rely on it? I guess CentOS 6 would be the oldest still supported distro.