That is absolutely correct but you could bend the join requirements to force the adoption even against the euro requirements criteria. You can even talk about skipping the line under some specific requirements.
The EU lets some of the members keep their currency because they are not that important to the gross number. Everyone knows the game they are playing but looks elsewhere. I am sure the Pound was always a problem and you have to consider again that UK never accept the Euro in the first place. I am pretty sure the way can be found to force the pound out and if not, they will be required to at least go the sweden way to keep it at the expense of “cheating”. It is a huge difference in political terms.
During Brexit negotiations it was cristal clear who had the hight ground and the UK had to comply to a lot of their red lines.
That isn’t an Issue, jet. But it could be in the long run.
The fact that the EU haven’t taken measures in that matter doesn’t mean it will not do it in the future.