I'm not a big fan of this metric. It isn't really any harder to fake liquidity than it is to fake volume. Sure, exchanges will have to figure out exactly what Coinmarketcap looks for in an order book, but this can be done. Just because there appears to be a buy order doesn't mean there is anything actually underlying it. If an exchange is willing to fake their volume, chances are they also will fake their orderbook.

The best metric I've seen for exchange volume is Coingecko's: https://www.coingecko.com/en/exchanges

Assuming you have a pool of accurately reported exchange volumes (not every exchange fakes volume), you can get a fairly good estimate for a site's volume based off its web traffic. Exchanges that have much higher volume than their web traffic suggests are consequently adjusted.