There's a thread discussing this issue:
https://forums.macrumors.com/threads/1807367/
And your post seems to confirm my suspicion. Some people are reporting that you are supposed to receive the $50 coupon regardless of what the trade-in value is. However, I'm pretty sure the rep was correct in saying that the coupon will only print out if your trade-in value is less than $200. I'm still waiting on some other people to confirm this.
I traded in an iPad 4th generation for $250 and I got the $50 coupon.