The discussion of the examples in Section 4.3 glosses over an important detail: IGEN will only generate the output shown there if the time pressure is eased by increasing the delay thresholds.
the choice of #<w :DRIZZLES> and #<w: POURS> over #<w:RAINS> in those examples is made with the thresholds set to force the utterer to always wait at least five time units before uttering the next piece of text.