If we have to send a winning multiplier via DM Then don't you think there would be less transparency as you will be the only person to know who had the highest multiplier.
I don't think any whale who is concerned about revealing his/her identity will participate in this 100$ giveaway. Most of the participants will be betting at very low limits just to qualify for this promotion.
So why not tell all to post their winning multiplier directly in the thread.