Nothing you’ve stated contradicts my point. You are also misunderstanding the chemistry in the passage you quoted.

Sulfur is commonly used to bootstrap exothermic reactions, largely based on its strong affinity for oxidizing metals with relatively low activation energy. In gunpowder, you need the potassium nitrate to release the “nitrate” part so that it can oxidize the carbon but for that to happen you need to offer potassium something more thermodynamically attractive to bind to. Charcoal (carbon) cannot serve that purpose. Sulfur provides that thermodynamically attractive outlet for potassium such that it will happily release the nitrate. Sulfur, in this case, would be termed a “sensitizer” but the mechanism of action is as a metal oxidizer. Sulfur/metal complexes are often used to bootstrap hard-to-ignite things such as thermites.

There are many metal oxidizer salts that become so sensitized in the presence of sulfur that they become dangerously unstable. In the case of potassium nitrate, it becomes more sensitized but still stable enough to be useful. For most real industrial applications, metal oxidizer salts have long been replaced with ammonium oxidizer salts which don’t have any metals to worry about.

The “sulfur-free” formulation uses an organic fuel (gunpowder does not), which provides alternative potassium sinks thereby obviating the need for sulfur.

> You are also misunderstanding the chemistry in the passage you quoted.

I’m not. I’ll give the benefit of the doubt we have a miscommunication though.

> Technically speaking, sulfur is an oxidizer in gunpowder, not fuel.

This initial statement makes it sound like you were claiming that the potassium nitrate is not the primary oxidizer in the combustion. It is the potassium nitrate’s role which is the oxidizer and not the fuel.

And yes, sulfur attracts the potassium, and aids in the reaction, but it’s role is also as fuel, which you incorrectly stated it was not.

Again, from wikipedia:

sulfur (S), which, while also serving as a fuel, lowers the temperature required to ignite the mixture, thereby increasing the rate of combustion. [1]

[1]https://en.m.wikipedia.org/wiki/Gunpowder#Components

And Fuel:

A fuel is any material that can be made to react with other substances so that it releases energy as thermal energy or to be used for work.

Sulfur itself can be burned. Potassium nitrate cannot. In fact if you add sulfur to potassium nitrate the sulfur can act as the reducing agent and you’ll get sulfur dioxide.

  4KNO3+5S⟶2K2O+5SO2+2N2

To categorize sulfur over the potassium nitrate in the role of oxidizers doesn’t make a lot of sense, which is how I interpreted your initial statement, perhaps wrongly.

Gunpowder (specifically black powder) works in a vacuum because of the potassium nitrate, not because of the sulfur. You can have gunpowder without sulfur, but it’s not going to work without the potassium nitrate.