Ah, I didn't argue that religion gives you a correct moral code, or that there is only one possible moral code. Religions are the result of memetic evolution so it's natural that there's some variety. All I argue is that science doesn't give you one at all.
Proving this would take at least a whole book, maybe more, but such books have been written many times in the past when science vs religion was more topical. Most people don't demand such a book because "science can tell you how to do it but not whether you should" is a pretty uncontroversial position that's been taken by scientists themselves many times, it's pretty much the starting point of the debate over whether Nagasaki should have been nuked for example.
Still, consider a sample moral problem: revenge. Christianity (to take a familiar example) has a very evolved take on revenge. It teaches that you shouldn't avenge wrongdoing (eye for an eye), instead you should "turn the other cheek". If everyone did this it has obvious advantages for stopping conflicts escalating into long term violence, but it denies people a sense of justice. So Christianity also has a whole lot of evolved stories about how that's OK because sinners will be judged in heaven by God and maybe sent to Hell if they've been bad, so you can turn the other cheek yet rest assured that justice will be done ... in the afterlife. Obviously all this is totally unfalsifiable, it doesn't attempt to be rational in any way but it's a really useful set of beliefs and stories for creating a more peaceful society.
If you take only science as a starting point, how do you derive this sort of moral code and - just as importantly - make it stick? How would you teach it to children? In fact you'd struggle even with the far more basic task of deriving a moral code that bans murder. You can't start with evolution because animals kill each other all the time; there's nothing unnatural about murder. It's the prohibition against it that's unnatural, but science is just the study of the natural world, so that's no help. See for yourself, try it. Come up with a rational first-principles explanation of why murder is bad without invoking religion, and which doesn't simply boil down to tautologies, circular reasoning, appeal to authority or other logical fallacies. I've tried - it's hard!
"if that were the definition of scientism then I wouldn't subscribe to scientism."
Right, but nobody explicitly subscribes to scientism. We don't even have such a word as "scientismist" because it's pretty much the defacto default religion of the modern population.
Scientism is perhaps the nastiest of all religions because it denies its own nature. Other religions accept what they are, but this one presents itself as pure rationalism even though it's not. It's very, very easy to accidentally become a follower of scientism which is why so many people are. All you have to do is use reasoning like this:
1. Science is a good way to determine what is true.
2. But I am busy and don't have time to do science. Therefore I will delegate it to professional scientists.
3. I don't have time to figure out who is applying the scientific method on a person-by-person basis, so I'll delegate that to scientific institutions (universities, governments, corporations etc).
3. I don't have time to verify that these institutions are actually using the scientific method. Actually I'm not even sure what the scientific method is, beyond the very basics. So I'll assume that if people are being referred to as scientists by other people who are also being called scientists, and the places they work have money, then they are scientific institutions.
4. I don't have time to actually read what these institutions write down in their holy books/journals, because a lot of it is written in Latin, but I am sure that it is Science and that The Word of Science is Truth, so whatever these institutions claim must be true.
5. I don't have time to actually find out what these institutions claim, but there's this noisy town crier in the middle of our village who says that the institutions claim X, and nobody seems to be objecting, so I have Faith that this must be The Word and therefore Truth.
5. I don't understand why these oddball heretics keep popping up who lack Faith in The Word as transcribed by the priests at the Institutions, because why would anyone doubt what is obviously and clearly True? Yes, scientists are merely men and may sometimes be unclear about the universal underlying Truth, which may even be unknowable to us mere mortals, but nonetheless it is important for community cohesion to have Faith and therefore we should banish these heretics as far outside the village as possible.
In other words, although people like to think they're rational, being rational is time consuming and difficult so most people delegate it. But - and this is where it becomes scientism - they don't recognize that this is what they've done. They conflate belief in a hierarchical Church-like system with rationality, and end up believing anything that people working at these institutions say even if it's obviously pseudo-scientific. And, they treat doubt in the sciencey-ness of the priestly class as terrible heresy; such heretics deserve to get their printing presses smashed up (cancelled).