That's true, but in the modern era the Catalan form of independentism (like the Scottish and Flemish ones) is an aspiration to be an independent country still tightly integrated within the EU as the overarching supra-national framework, so there wouldn't really be a goal to put up border walls or be capable of militarily defending the border, any more than Luxembourg does either of those things. The hope was that as nations integrate closer into the EU, the EU picks up many of the traditional responsibilities of a nation (external border, customs, trade agreements, etc.), making it more viable for regions to detach from countries that themselves partly function as federations (like Spain, the UK, and Belgium) and just become direct members of the top-level federation.

Now it's quite possible that political or procedural reasons (like the ease of vetoing new EU members) mean this is unlikely to happen, but it's generally the envisioned route that independence supporters in these regions have.