He has enough SP/MVESTS to vote in 20 new top20 witnesses with his stake alone. That is a fact.
Your logic is flawed because not everyone agrees on the top 20 witnesses. The votes of the community are spread out across 200 witnesses and not everyone is voting and not everyone has voted 30 witnesses.
If you wanna see the math read one of my latest posts. It does not even cover all the details if he really has 70M STEEM. 60M is more than enough if his votes are given to 20 new witnesses.