Third party software

[Packetdancer]

Third party software = anything that modifies the game in any way.
Discord and voice clients does not. So its okay. They do not specify this because they shouldn't have to.

Discord does in fact potentially modify the game process if you have its in-game overlay enabled, the one that shows you direct messages and the 'who is in voice chat/who is speaking' widget; that hooks DirectX directly.

And if I remember correctly, even Yoshi-P has noted that yes, Discord technically counts as a third-party program. (Though if I'm remembering right, he also had a slightly pained expression on the livestream where he did so, as though he were silently asking himself "Why is this a question? Do people actually think we'll ban Discord, or are they just being pedantic?")

Except this statement simply isn't true because a truly vanilla experience means not using ACT whatsoever. This, in turn, means you can't maximize your DPS for something like Dragonsong where the enrage is so tight a single DPS death can be the difference between getting passed Estinien or wiping to Revenge of the Horde. And this is from high level players pumping out as much damage as possible. Which they only know how to do because of the aforementioned third party program and a website to parse that data.

While ACT is undeniably useful, both for improving your own efficiency in a fight and for figuring out where your static as a whole can improve, it is not required.

The optimal rotations (and ideal GCD for a rotation) can be determined mathematically outside of the game, using the information on tooltips. Arguably, it's probably easier to figure out the optimal rotations in a purely-mathematical sense outside of the game; the lack of ACT would not prevent the Balance -- or SaltedXIV, or AhkMorning, or whatever else you prefer as your combat-efficiency reference -- from posting optimal rotations and melds.

(Now, whether Determination or Direct Hit is a better meld for certain jobs is something that potentially does need to be experimented with... but while ACT makes that undeniably easier to do, it's not required.)

ACT definitely makes it possible for a group to fine-tune their efficiency far better than if they didn't have it; it's a lot easier to spot "Oh, hey, the NIN has drifted a bit, so their burst no longer aligns with raid buffs." or "We're losing the end of that burst window there, we might want to delay it a bit until after this mechanic so we don't interrupt it to move." And it's certainly a lot easier to tune your own combat efficiency if you have ACT (and can upload logs, and compare them to people who perform very well in your chosen job to see what you can do differently to improve).

But I have a friend who happens to be in a static of all console players; this wasn't deliberate, just by pure chance they all happened to play on console. As a result, they don't even have the option to use ACT, and they still clear content. I won't lie, they've expressed before that they wish their static did have the option to use ACT in order to more easily see where they need to clean things up. But the simple fact is that since the entire group are console players, they have to make do without.

ACT is undeniably an extremely useful tool to help you dial in your performance more quickly, whether individually or as a group, and so is very useful to clear content; without it, you will clear content less efficiently because figuring out what your group needs to change in order to beat that enrage can take a bit longer.

But "less efficient" doesn't mean "impossible".