Often courses that use recorded audio instead of computer generated audio don't have a slowed down version. I am not sure why but I am sure a contributor may know.
I'm not a contributor, but with computer-generated audio, you just need to tweak a setting and you can have a file of any speed. With human-recorded audio, this doubles the number of recordings they need to make and significantly slows down production time.
I'm pretty sure you also can have a file at any speed with human-generated audio, with an algorithm. The only thing is that they didn't planned it. I noticed a lot of "slow down" button, and stuff disappears when they revamped the site, complaining that it was too heavy on their servers.