First of all the original KMS event was for 200 as well, not 250 (assuming i heard this correctly). Looking at GMS' history it's unlikely they would change this.
Secondly, reaching 250 is such a ridiculous requirement for such a short-lived event. It should have raised red flags and deterred you from purchasing any nx coupons or losing sleep over it. Your inability to stop for a second and think about it rationally is nobody's fault but your own.
I apologize if i sound harsh, but i do not look kindly upon this kind of sense of entitlement. From day one people were questioning the 250 requirement. You knew what you were getting into. You got the best legion block so be happy and be proud that you made it all the way to 250.
Whether the false info at the event page was a mistake or done on purpose is irrelevant. Everything nexon says or does should be taken with a grain of salt.