Here's the thing. You said a "Croatia is Serbia."
Is it in the same culture group? Yes. No one's arguing that.
As someone who is a scientist who studies Serbia, I am telling you, specifically, in science, no one calls Croatia Serbia. If you want to be "specific" like you said, then you shouldn't either. They're not the same thing.
If you're saying "Serbian culture group" you're referring to the grouping of South Slavs, which includes things from Slovenians to Bosniaks to Bulgarians.
So your reasoning for calling a Croatia Serbia is because random people "call the slavic ones Serbians?" Let's get Albanians and Slovaks in there, then, too.
Also, calling someone an Aztec or a Chinese? It's not one or the other, that's not how culture grouping works. They're both. Croatia is Croatia and a member of the Serbo-Croat family. But that's not what you said. You said a Croatia is Serbia, which is not true unless you're okay with calling all members of the Serbo-Croat family Serbs, which means you'd call Russians, Bosniaks, and other Slavs Serbians, too. Which you said you don't.
It's okay to just admit you're wrong, you know?
Circlejerk copypasta with...cultures...yay