Even the quote you use is intentionally incomplete. The crucial part of the parent quote is 'is determined by how long you "hold the key"'.

Go play with a synth, or even look at an ADSR envelope tutorial, and you'll see you were wrong. And not just wrong, but condescending and wrong.

There’s a gate signal, typically activated by holding a key (though in a modular synth, there are many other potential gate sources). While the gate is open an ADSR envelope progresses from Attack -> Decay -> Sustain. It then remains on Sustain until the gate closes, at which point it enters the Release phase. So the amount of time it remains in Sustain is dictated by the gate signal. Notice there’s no G in ADSR, because the gate doesn’t come from the envelope.

What you’re describing is Hold, which some envelopes (AHDSR is one popular flavor) can do. Many Elektron groove boxes have hold stages on their envelopes.

In AHDSR, an open gate goes from Attack into Hold, where it retains its value for a set period of time after the attack, and then goes into the Decay phase and continues from there.

There are plenty of other kinds of envelope, and things that live somewhere between LFOs and envelopes called a function generator, which is often an AR envelope that can be looped. Then there are complex many-stage envelopes that were especially popular when digital synths were first coming onto the scene.

I’d also add that the description you gave of a synth architecture is generally considered East Coast synthesis. One or more oscillators going into a mixer/VCA, and then into a filter and possibly some effects is a very popular architecture made famous by Bob Moog. But there are other forms like West Coast synthesis where instead of having filters, you run gentler wave forms like a triangle through a wave folder, and/or a complex oscillator where you have a pair of oscillators that can cross modulate one another. So where East Coast synthesis takes harmonically rich waveforms and cuts harmonics away with filters, West Coast synthesis starts with harmonically tame waveforms and adds harmonics through various flavors of FM, wave shaping, etc.

Then you’ve got samplers, granular synthesis, physical modeling, additive synthesis, and a bunch of other types as well. The East Coast architecture is popular, but there’s a lot more out there.

> FM is Frequency Modulation which is what you can do with oscillators and the above mentioned envelope points.

This isn't what FM is. FM involves using one oscillator output to modify (modulate) the frequency of another oscillator. ADSRs can be used to shape the degree of modulation over time, but the sound isn't shaped through the ADSR in this case; rather the ADSR controls the sound generation stage by modifying the frequency modulation effect in the time domain.