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By my reading, that parenthetical comment is implying that the author initially considered "XXX" as a placeholder for the letters coming after a literal "XXX" in the DEC source code, but couldn't use "XXX" as a placeholder because it's also being used literally as the prefix, so "XXXXXX" would not demonstrate what the author was trying to communicate (that the DEC code in question had many instances of the letters "XXX" followed by three other letters).

Perhaps XXX001, XXX002, XXX003 and so on?

The standard placeholder would be to use "XXX" instead of "YYY" but "XXXXXX" is semantically ambiguous in this context.