"Probable considerable reduction of BB gauge fill rate" How is that a totally different in game description?

Probable, meaning that there's a small chance. That's the 20% activation part. Considerable reduction of BB gauge fill rate is the resulting effect of that activation, and since it's "considerable", you can expect it to be a decent amount of BB fill rate reduction. It wasn't gumi's fault that the "in game description is totally different", it was your fault for not being able to interpret the message correctly.

I'm not saying this to hate on you or be a dickhead, I'm just stating facts. But regardless, you should write a support message and hope that they'll re-trade the elgif to you. But if you say that the effect is totally different from the in-game description in your message, then good luck with that.