I find this untrue. historically, unions represent jobs that are most at risk from replacement - e.g. by strikebreakers. Which is opposite end of the spectrum from 'hot demand and well compensated'. In fact, unionization are spurred by the opposite of these things.
You've missed their point a bit here. It wasn't that the most common time to start a union is when you have leverage, it was that that is the best time.
Software workers could achieve huge results if they collectively bargained, and the relative scarcity of labor means that if they unionize it will be comparatively difficult to skirt those unions.
Also, there's not knowing what you've got until it's gone. Software workers have the power to organize now, but there is good reason to believe that, for one reason or another, that power will decrease at some point in the future. That has happened to practically every industry in history. Software workers would be wise to organize when they can, instead of waiting until they are forced to.
