Presumably in the sense of "can only play one note at a time".
At a guess, the whole business of spinning up a new instance of the sound generator for each new note that's played, (or running a fixed number of instances, and dynamically allocating new notes whichever instance will be least disruptive, aka "note stealing"), and the other business of doing modulation (such as vibrato) that needs to be applied consistently across all instances of the sound generator... remains to be implemented.
In other words, it looks to me like Faust's focus at the moment is on describing the processing in the audio signal path. Polysynths require a whole bunch of cross-cutting management functions, outside of (and between) the (multiple) audio signal paths.
This is correct interpretation, you can have stereo output but your instrument can handle only one note at the time.
There are plenty of modern synths that have "mono" or "solo" option. If you play two notes at once, the resulting tone will slide from first note to the second. This is obviously very useful for playing melodies and keyboard solos where polyphony is often not desired.
BTW there are ways to use multiple mono-synths as a poly-synth, depending on VST host.
At a guess, the whole business of spinning up a new instance of the sound generator for each new note that's played, (or running a fixed number of instances, and dynamically allocating new notes whichever instance will be least disruptive, aka "note stealing"), and the other business of doing modulation (such as vibrato) that needs to be applied consistently across all instances of the sound generator... remains to be implemented.
In other words, it looks to me like Faust's focus at the moment is on describing the processing in the audio signal path. Polysynths require a whole bunch of cross-cutting management functions, outside of (and between) the (multiple) audio signal paths.