Comment by cesarb

> Does this mean that any GPL project could reuse source code from this game, under the logic that it all inherently must be GPL licensed?

No, this only means that the game developers are violating the copyright of the copied code. The GPL is not "viral"; including code under the GPL in a project without following the GPL only means that the GPL doesn't apply to it anymore ("[...] and will automatically terminate your rights under this License"). Since there's no longer any license allowing it, the default restrictions of copyright apply, and the result cannot be distributed without the permission of whoever holds the copyright to that piece of code under the GPL.