Their branding has been so bad that this statement—as bad as it is—is actually incorrect because the truth is
even more convoluted.
HBO Go was for HBO (cable) subscribers to access to HBO cable content they were already paying for, but through a mobile app at no additional charge.
HBO Now was a distinct streaming service that was available to non-cable subscribers, and provided access to HBO cable content without an HBO cable subscription.
HBO Max was a distinct streaming service that was available to everyone, and provided access to HBO cable content + HBO Max exclusives with or without an HBO cable subscription.
There was a brief window in 2020 where HBO Go, HBO Now, and HBO Max all existed alongside each other as distinct services that someone could be subscribed to, before HBO Max (later Max, later still HBO Max) eventually ate them all. I might enjoy HBO Go as a cable subscriber, and then additionally subscribe to HBO Max so that I could enjoy HBO Max exclusives, while my neighbor only did HBO Now so that they could enjoy HBO cable content without having to subscribe to cable.
Loads of articles like this one popped up at the time to help explain the mess.
Clear as mud?