Products on the App Store can be sold at a different price, companies often do this to recoup some of that 30% App Store commission.

Edit: this is a fact, though feel free to downvote. Apple tries to hide this from the user by not letting publishers mention that the app or service is cheaper when purchased outside of the App Store or Mac App Store.

I must be misunderstanding this. Since Apple doesn't support sideloading, how is any product offered both on the App Store and somewhere else, let alone for a different price?

Some apps are sold for a different price on the App Store and the Mac App Store, than if you'd buy it directly from the developer or a store on a different platform. On iOS you can obviously only buy an app on the App Store, with some exceptions.

You can buy subscriptions outside of the App Store and still use the app.