that is not tx volume, bitcoin transaction volume should be measured on number of transactions not their value in USD specially when you are comparing it over the long term where price has increased dramatically which is the case here. not to mention that we are talking about bitcoin not USD so reporting things in dollar terms is silly on its own to begin with.
I think he thought that by adding the values in USD it will attract more views since people we the difference better and faster when values are written in fist currencies. I definitely wrong to compare values in USD when we are talking about bitcoin but not completely wrong in my opinion because we can't assume everyone that had bitcoin in 2018 has hodl it untill now. Therefore money are coming in and out while people make decisions based on the price of the moment they are buying selling.