Not sure I'd agree. The only definition of 'goodness' that's really relevant is 'that which allows life to thrive'. A creator could create any number of species for whom 'goodness' is different - it's counter to goodness for humans to be set on fire (therefore being set on fire is 'bad') but a creator could make a species that thrives by being set on fire, therefore it's 'good' for them.
But that wouldn't change the fact that 'goodness' is still fundamentally whatever allows a particular species to thrive.
At best you might have some all-knowing entity with complete knowledge of exactly what the perfect recipe for human thriving was, and therefore knew what ultimate human goodness would be, but that's still besides the point that goodness is what allows for thriving.
Think bigger. Think about a being that can change the rules of how life works and what it is. Or, even make rules for creating sentience that don't depend on living systems.
The limit isn't my imagination or yours but possibility itself, whatever that boundary really is. Think about a universe that has different entropy. How energy and matter work. Anything.
Good can be defined differently according to the whims of such a creature. The idea that it would generally make the creature to FIT the definition doesn't seem like that much of a constraint.
You could even create a creature that thrives by trying not to thrive and self hating everything that is truly good for it and putting it in an opposite universe where every harmful action the creature would take to commit suicide made it instead thrive according to that God's plan.
If an entity tried to declare that something harmful and dangerous that detracted from human thriving was 'good', what exactly would be the motivation to care about what they have to say?
That we are the ones defining "good" according to our own experience seems evident to me, but I am giving credence to the idea that there could be a God that wanted it that way (for the sake of argument). So, I don't see how the dilemma resolves anything morally with regards to God, we can't demonstrate God's views on the matter aside from what we already know about the universe.
So, since there are no real constraints on the powers that God can have, I have no way of saying that God doesn't indeed define Good. The dilemma makes the mistake that "the good" is objective, and in a case where God can do as it wishes and has the power, it should also have the power and imagination to define it as it so chooses.
If God had created a universe where we would see good one way and it would see it another would put us at odds with such a creature and we would call it "evil" instead, and we quite funny enough would worship "the good" rather than the real god of our universe. We tend to never really imagine this possibility but it's also impossible to rule out.
So, simply put, God as a concept can't be too absurd to exist, our notions of absurdity are no standard for something we don't understand. A rationalist perspective on "the good" and it's relationship to "god" are no dilemma if we don't know how Gods operate and what they are, and they are free to be of any magnitude of power. It's simply sets us upon a fools path to grant the premise that God's must have a rational and direct relationship with our morality and reasoning.