When you put it that way, it's silly, but I'm sure he meant to restrict the requirement to software source. That's not silly. A master tape and artist's sketch is not software, but every example in the original post (SVG, wav, docbook) is software.
The reason to make a special case of software source is that it can be distributed just as easily as the software object code.
Remember that the free software movement is primarily interested in separating out that software whose publisher withholds source for the sole purpose of preventing people from modifying the software. Someone who holds back SVG source is in the same category as someone who holds back operating system source.
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