Just as Free Software is, effectively, whatever the Free Software Foundations says it is, open source software is whatever the Open Source Initiative says it is.
Perhaps it's that easy, but rms is referring to comments made about the GPL (which is definitely aligned with free software, not open-source) and which were made a year or two after the OSI pinched/borrowed Debian's Free Software guidelines and came up with their initial definition.
So, at the time Ballmer was a) definitely talking about free software by mentioning GPL, and b) probably not particularly interested in, or aware of, the split by the 'open source' proponents.
So, at the time Ballmer was a) definitely talking about free software by mentioning GPL, and b) probably not particularly interested in, or aware of, the split by the 'open source' proponents.