I don't know how I did manage to do it.
But now I'm struggling to understand how exactly was I able to do it, because in 79% of cases I am greeted with this message:
and in another 20%, this faked "improve" goes through (while I don't change completely anything) and I can haz a binding approval vote :D
So it's definitely a bug, but I can't find the exact way of reproing.
Note: earlier there was actually a bug which allowed me to approve an edit by clicking improve and submitting with no changes, then it was fixed and for some time the message I've shown above stopped me constantly, and now there is another bug