Maybe, but the OP, presumably a regular person just like you and me, seems to be confused by this. And if you look at the comments page on pretty much anything to do with EU jurisdiction, you'll find that's just one of many, many mistakes regular people make on the topic of which treaties apply to which countries.
So, yeah, it's not rocket science, but it's not as obvious as "people in France are subject to French courts" either, or as "all countries in the EU share the exact same treaties".
More to the point, it's a complexity that politicians love to exploit to spread confusion or outright lies, when it suits them. Therefore, I think there's merit in reminding people to double check exactly which treaties apply to which countries, when trying to understand European affairs.