Yep, I agree with the other folks that this was probably a misunderstanding.

I have actually used this a few times to get a free E+ seat, by purposefully not choosing a seat on flights that I doubt will fill completely. I think I'm 3 for 3 on getting a free E+ seat, but it's always been on regional jets When I've done it, I've always gotten a seat assignment when I checked-in online and the assignments were in E+. The one time I got an E+ aisle, I was able to get a United Club agent to switch me to a confirmed E+ window without needing to pay.