Most likely it's because it was a scraped site: no human eyes involved. They know that saying "audio book" gets clicks, so it's in the script, but they don't check whether any individual book actually has an audio version.
Audible has an affiliate program which means that the actual audio version may very well be for sale on the site. If you are an Amazon Associate (and you should be if you have your own author website so that you can get the commission in addition to the sale of your own books) you will see the link to audible.com in the lower left hand sidebar of your associates page listed under "other affiliate programs".
At one time, Amazon aided in the "scraping" of content by releasing their API for use in developing plugins that allowed users to find products by type so that they could create a webite about a certain topic and then offer relevant products. There were, and probably are still, some sleazy uses for that method, but I know someone who has a passion for gardening and used one of these plugins to help support his gardening blog.
He was spending a fair amount of money on the blog to test organic fertilizers and such and then report his results and he hired out a photographer to keep things looking good. He has since closed down the site because he wasn't able to make enough income to keep it going even though a lot of people visited the site. It is a shame because he provided excellent, balanced information.
Wonder if ACX promotion sites doing something similar to Bookbub would be the next big thing.