The claim that matters is that people want independence at all. Why isn't important, self-determination is.

The context in which things have been happening matters. Catalonia asked for fiscal special treatment like the one that other regions have for historical reasons (whereas they don't have such history). So the context is important.

Catalonia could have obtained a similar system when the constitution was written, but declined to do so.

Plenty of counterexamples, including the joint independence movement among 13 English colonies in North America (which, unlike decolonialization in, say, India, wasn't the historically independent subjugated locals revolting against the foreign colonizers, but largely the imported ruling class revolting against the homeland that set them up and displaced the locals.)