The moon base example I think makes the argument very clearly. If you have an economy which produces nothing then it has a GDP of 0. If the increase imports for whatever reason, their GDP is still 0, which means that imports doesn't subtract from GDP, otherwise their GDP would be negative which is nonsensical.
But all this is sort of beside the point because arguments from accounting identities are almost always nonsense.
It's technically true that imports don't decrease GDP, but that's only true if there's no substitution effects. But for most goods substitution effect does exist, and therefore we should expect a GDP drop from imports.
This equation is tautologically true. It is not some “almost always wrong” blah blah. People here know almost nothing about economics and are vulnerable to the most specious of arguments.