Earlier quoted context omitted.
> People say the US constitution doesn't allow secession of a state (as opposed to a territory) without a constitutional amendment. Congress granting a territory (or state, directly) independence isn't secession, whether or not it is allowed. Secession is unilateral, grants of independence are a different thing.
I don't know if that is the key difference here. Madrid insists they couldn't allow Catalonia to become independent even if they wanted to. Many people in Spain outside of Catalonia are resolutely opposed to Catalan independence, under any circumstances. By contrast, most people in UK outside of Scotland don't really care, and even the vast majority of those opposed to it would be willing to accept it if a referendum…
Which may or may not be a correct interpretation of the Spanish Constitution, but what the US Constitution does or does not allow regarding either secession or Congressional grants of independence is not really germane one way or another. They aren't even products of the same legal tradition such that analysis of one might be illuminating on the other.