First, Quoting a number like “$19699.46” to seven significant figures when your data’s got a 5% spread would get your high school physics teacher slapping you upside the head. It’s entirely deceptive. It should say something like “$19,700 plus or minus $500 depending,” and that line graph should be a thick grey bar.
The question is does the 5% spread change my buying price? I need to buy a bitcoin. Will coinbase honor the price when order is placed ie if it shows $19700 then will it let me buy 0.1 BTC at this price? Similarly, will it allow me to sell them back at whatever price they show on screen? If yes, how does it matter?
Sure, if coinbase cannot honor the price and let's in a huge slippage [1], then that is a concern. Though in most cases where they see markets moving too fast for them, they have routinely shut down trading.
Second, in stock markets it is not about smart order routing but NBBO or nation best bid and offer. Brokers need to route orders to best price available else they are liable for damages. There are no such rules in bitcoin. And as the article notes, there are issues in getting the best price available due to transaction times. In which case, coinbase can set whatever price they want. But if they honor it, there is not much to talk about.