The author seems to be getting at that point: these two "Apple TV"s are more than two products.
Assuming for a moment that the hardware devices can be a single product, the apps just cannot be: each of them has wildly different behaviour on different apple devices, with different features, and different functionality depending on the presence of 3rd party apps.
As an end user I found this confusing at first, frustrating when I understood what was going on, and now I just don't use the Apple TV apps, even on my Apple TV.
I guess this is subjective, but I don't see the behaviors as "wildly" different.
In all cases, the Apple TV app is where you go to access the Apple TV+ service and as much other available content and video services as can possibly be presented to you on the given platform. The fact that this is different depending on your device and subscriptions is a little bit weird, but it doesn't change that much, at least for me.
Assuming for a moment that the hardware devices can be a single product, the apps just cannot be: each of them has wildly different behaviour on different apple devices, with different features, and different functionality depending on the presence of 3rd party apps.
As an end user I found this confusing at first, frustrating when I understood what was going on, and now I just don't use the Apple TV apps, even on my Apple TV.