Amazon Prime was originally an add-on to regular Amazon accounts that provided expedited delivery as a flat rate subscription. Later, Amazon got into the streaming business and bundled access to films and shows into the Prime subscription. For a while it was kind of a useless perk since the available content was old or low quality.

Later, as Amazon acquired more popular content and then created an in-house production studio, they added the ability to rent or "purchase" (rent) streaming video content on a per-episode/season/film basis without requiring the full Prime subscription service.

This was designed to address Amazon customers who were primarily (pun intended) interested in the video content, not the physical goods.

I think he’s saying you can subscribe to Amazon Prime Video via in-app purchase.

Yep, not the full Prime account, but a cheaper video-limited one.