To know for sure we'd have to ask the GPL's original authors, but my reading of it was that the term was added to ensure that the right to produce modified works could be meaningfully exercised. In particular, it was to preclude distribution of "source" that was obfuscated (e.g. variable names changed, no whitespace, no comments, replace control structures with equivalent gotos) or which was already compiled (e.g. as "binary blobs" which form part of the resulting program/work).
I don't think anyone was thinking about VCSen back then.