Monochromatic just means that the light is centered around a very narrow band of wavelengths (there is an infinite number of wavelengths between, say 400nm and 401nm).
I think 589nm is a similar DPSS process to green lasers. DPSS lasers are monochromatic because although the process involves different wavelengths of light, the end product is light of "one" wavelength. Not two or three combined.
589nm lasers are used to calibrate telescopes for atmospheric turbulence by creating a "guide star." If the laser was just a yellow and green beam combined, it would not work.
594nm or 593.5 or whatever is produced by sum-frequency generation. I won't pretend to understand the physics involved but from the bits I can pick out, the photons are effectively "smashed" together. This is not the same as combining two beams like in a projector. It actually does produce one wavelength.
If you pass a 589nm beam or a 594nm beam through a diffraction grating or prism, only one beam will exit. If the beams were just combined "colors" diffraction and refraction would separate the beam into it's constituent parts.