Hasn't really established that at all, there's plenty of tradition of using chorizo in paella. Mexican chorizo is, in my experience, mostly pretty uniform, but there are varieties (including a green one stuffed with cilantro). Spanish chorizo is much more variable; I've found several quite distinct types locally, and that's just the tip of the iceberg.While this thread has established that chorizo is not used in traditional paella (thank you El Marin and others), as an aside – and for the hippies out there and others who tend towards “arroz y otras cosas” – I don’t think it’s been said yet, but there are at least two types of chorizo: Spanish chorizo and Mexican chorizo. They are both good but vastly different.
I see the OP is from London so they probably used Spanish chorizo, but in western and southern US it’s pretty hard to find Spanish chorizo, unless you seek it out.