With a policy on acceptable addons, then there is no gray area. If a change is unacceptable it isn't allowed
You're exactly right. ToS can't perfectly distinguish. The line I've laid out focuses on actions that alter the value of other users' product or service because that is the most defensible position.
I have brought SDKs. Without goin super technical, an SDK must roughly exist already in the client for it to communicate. Working with Dalamud devs or starting an open project, addons would HAVE to use the Official SDK. Long story short, it gives devs the ability to very easily control addons at a feature level. For example, all plugins could be disabled in PVP. If any exposed functionality is abused it could be disabled very easily, by a toggle and all plugins using it would cease to work
There is a problem, despite what many assert, on the business side. If recent drama over addons forces them to come down harder on all addons suddenly you have a much faster growing pool of potential class action litigants.
Imagine 100,00 accounts suddenly having their already long spent game, expansions and sub fees reversed -- that's the ceiling for damages that will be negotiated against. They don't need to have a rock solid argument that wins, they only need good enough to not be thrown out. Then you're likely settling out of court - we're looking at likely 10s of millions of dollars
There are reasons they don't just go ban silly on everyone now.
There's no perfect anti-cheat solution. There's no winning an arms race in any cyber security; best you can do is make the playing field on your terms. By changing bans to be content-specific, the user retains their ACCESS so the devs and good players don't have to suffer the effects of the legal world invading FFXIV
Considering all that, the points I've outlined take away any argument by cheaters. Any plugin that does NOT use the official SDK must, by definition, be reverse engineering the game to maliciously devalue the subscription of users following the ToS. That puts SE in a position to sue those who make unofficial plugins or reverse any action against them.