what exactly is the point of the issue of the wasteland survival guide that "permanently marks diamond city on your map" if that already happens like 30 minutes into the game
It's only useful if you immediately ditch the main quest and start exploring
AND happen to go south rather than east or south-east
AND happen to stumble upon Gorski's cabin and enter it.
I believe it is a leftover from an earlier design of the game, perhaps where Diamond City's location was given much later.
The wording is also peculiar too, stating it marks it "permanently", almost as if there was a chance it would only be temporarily marked, even though that is never a thing that happens.