It’s in the summary:

> We quantify the wash trading on each unregulated exchange, which averaged over 70% of the reported volume.

That's just the list of exchanges, wherein Coinbase is listed as a regulated exchange. Compare that with the text of the abstract:

> We quantify the wash trading on each unregulated exchange, which averaged over 70% of the reported volume.

See how the 70% figure applies strictly to the unregulated exchanges?