Queue for imperial city, use the exit door there. when you're in a delve you're no longer in cyrodiil... Although this sounds like a really bad way of handling it, should be a way to travel to your own home or wayshrines in instances like this.
This has been a problem for years. Never go into a delve in a poplocked campaign or you won’t be able to get out. You can relog or as mentioned above, queue for IC. Then you have to go back into the cyrodiil queue.
Battle not with monsters, lest ye become a monster, and if you gaze into the abyss, the abyss gazes also into you. ~Friedrich Nietzsche
This is just ridiculous this hasn't been fixed. If you go into a delve in Cyro, you should be able to get back into Cyro.
Suggestion: that there is a 10 minute limit for being in a delve after which it gives you a 2 minute warning that you'll be pop locked out of Cyro. If the time expires, then you are pop locked out, but until then, it holds the place for you to be in Cyro.