The article doesn't detail it fully, but a lot of electricity wholesale markets are structured like this:

• For each time block, the utility solicits bids to supply electricity
• The lowest bidders win, and every bidder gets paid the price that the highest winning bidder bid
• This "clearing price" then determines what the utility charges retail customers

So until you get rid of gas generation completely for some time slots, the clearing price that everybody pays is set based on how much it costs to burn fossil gas to generate electricity, even if, say, 90% of generation happens using wind and solar. As a result, all the benefit of wind and solar goes to the people owning the generation capacity, rather than retail utility customers.