No, historians do not know for a fact that Goths came from Gotland. That's just an easy and traditional explanation for their origin, based on accounts written by medieval "historians" who made up most of the things they wrote and one single coincidence between the words Goth and Gotland (and also 10 million other places with similar names), but it's highly debatable and not unanimously agreed, not at all, amongst the academia today. The consensus, roughly, is that they came from somewhere, and that somewhere could be a lot of different places, maybe even Gotland, but there's not enough evidence to conclusively prove any of those origins.
So, we have 2 PDS devs catiously stating that the origin of the Goths is debated and they could have come from different places, which is actually a pretty good version of what historians think about the issue, and then we have an overreaction by people who base most of their history knowledge on Wikipedia. Don't be so fast on jumping to correct someone when you're not absolutely sure that you're right.