That is listed under "unofficial sideloading methods". A more accurate title would've been "Does iOS support sideloading yet?" but I wanted to keep the domain name as short as possible :)

The Apple Developer program is not intended as an option for end users to enable sideloading on their device, even if that is a side effect of joining it. It is only intended to allow developers to briefly test new builds of their own apps in a limited capacity before uploading them to the App Store (or third party stores in the EU). Apps "installed" this way expire after a certain length of time and you must ask Apple's cloud service for a new certificate each time that happens in order to keep using them. You're still tied to Apple indefinitely this way. If your developer account is terminated for whatever reason, or Apple decides to increase the price such that you can no longer afford your account, then suddenly you no longer have sideloading, and you no longer have access to any of the apps you previously sideloaded.

Therefore, I lump it into the same category as jailbreaking -- yes, you can argue that the existence of that means iOS already has sideloading, but it's not officially supported.

Sidenote: You don't need to spend $100/yr if you want to go the "unofficial sideloading" route; AltStore (Classic) is available for free: https://altstore.io/