One of the assumptions of the view that price will double after the block halving is miners are currently selling ALL of their mined coins. If they do sell all of their coins now, they will have less coins to sell. Hence selling pressure will ease and price could go up if demand remains the same.
What if miners aren't selling all their mined coins now? If they are only converting just enough coins to cover their fiat running costs, then they will convert exactly the same amount of coins to fiat because running costs will not suddenly jump after halving. So, neutral effect on price.
Who has inside information on what miners are doing now? Are they selling everything?
They can't be selling everything, you don't need insider information to understand this. Miners are not aliens, they are just like you and me. Do you have some bitcoins in your wallet? If you don't, then I do...