Knockdown and Knockout are not attacks
They are Conflict Tests (p. 29):
Conflicts tests are used most commonly in combat, warfare, and intrigue. A conflict test is always used to resolve anything that would function as an “attack.” An attack might be a swing of a sword, sneaking past a guard, or using wiles to seduce a noble; effectively, anytime you would “do” something to someone else, you roll a conflict test. Unlike a basic test or competition test, where you are testing against the challenge and complexity of the action attempted, a conflict test pits your ability directly against your opponent. The Difficulty of these tests is your opponent’s Defense.
The bolded part is probably what caused this confusion. When introducing the rules, the system considers an "attack" whenever you roll against an opponent. But that is a more general rule, trying to explain what is a conflict for a first-time roleplayer, and not specifically related to combat. In combat, you are limited to one attack per round, but using the rules within the combat chapter, which says:
There are several forms of attack as follows. You may make only one attack in a round, regardless of action it requires; to attack multiple opponents at once, use the divided attack or two-weapon attack options.
And then it lists those options:
- Standard Attack
- Divided Attack
- Two-Weapon Attack
- Combining Attacks
Standard Attack is simply the default and most common attack in combat (p. 163), but as pointed out by the Assist action, not the only form of attack. You have other attack options, like Charge, or advanced options (the problem here), like Reckless Attack, which is specifically named as an attack.
Knockdown does not say it's an attack. The description simply makes it clear that it is a conflict test:
Using brute strength, you send your opponent crashing to the ground.
Roll an Athletics test against your foe’s passive Agility result.
Similarly, Knockout is also a conflict test:
A well-placed blow can send your opponent into unconsciousness. You
may only use this maneuver against an unaware foe. Roll a Fighting
test against your opponent’s passive Endurance result.
But neither of them are attacks, as the combat chapter defines them.
But Disarm is resolved as an attack
Disarm, on the other hand says:
You may attempt to knock an opponent’s weapon from his hands. Resolve
as a standard attack except your Difficulty is equal to your opponent’s
passive Fighting result.
Since it's resolved as the standard attack, it should be considered an attack as well. Personally, I wouldn't classify a disarm attempt as an attack, especially considering how they are usually followed by an attack, but I can see why some GMs would consider it an attack and not allow another attack in the same round.