He often does both. There are reasons the charts are compared to USD, as that is a steady currency to place holdings in the case of a bear market. Many many readers have expressed their interest in the USD charts specifically. When they are different enough, he covers them both. Generally, if not, most pairings are heading to the same place with a slightly different path.

When BTC has a steady price that you can walk away from the markets and the blockchain and know exactly how much money you are carrying in your BTC wallet, and that won't change tomorrow, that's when it would be most appropriate to adopt a BTC comparison as your final go to.