The actual price difference is currently huge between exchanges because of increased volatility that is unprecedented in known history of bitcoin (10k variation in one week. Wow). So it is a little too much to say "it is already 21k" because "it is already at 21k at bithumb" and to be more exact "there might have been only a single transaction at bithumb at 21k. Personally, I use the price metrics from coinmarketcap or coindesk, those services show the price averaged over exchanges, and are much more truthful than a single exchange price.

What is interesting, that with current volatility those two services differ widely between each other too