I don't get it... How did this happen?
You are apparently allied to whoever Netherlands broke free from, but that event should only pop up if you are rivaled to that nation.
Only possible scenario i can think of is that they broke free from a HRE-member you are rivaled to while being allied with the emperor, who got called in as cobelligerent...
As long as you are allied with the emperor, there is no real chance for you to get hre territory anyway, beside some rival shannanigans or attacking an ally etc.
Still buggy that you get that event while at war with them. Wonder what happens when you accept the alliance ^^ guess auto white peace.
Probably worth a bug report though.