I think this point is really key, more than the general idea that "there isn't really a short supply of good programmers." It's imminently easy for a rich capitalist to tell the difference between a factory laborer who works hard and one who does not, than it is for them to say the same about an engineer; the evidence is plain as day in front of you. Longer hours, more sweat, quantitatively more physical tasks done, period.
For engineers of most sorts, the signals are actually counter intuitive, a lot of the time. There's the easy cliche story of two engineering teams tasked to the same project (not necessarily even software only). Team A has a slow and steady road map, no drama, everyone leaves at 5pm, they never make any noise, mostly hit their deadlines, even if the deadlines are unambitious, etc. Team B always is fighting fires, huddled around someone's desk talking, always working overtime burning to make aggressive deadlines that are never quite reached anyways, always communicating with the wider company about status of issues and problems, etc. Which is the more effective team? Team B will definitely be "known" more in the wider company, and might have more individuals who respect them and think they are hard workers. If you took a "peer survey," Team B might actually still come out ahead. But they may still be a much less effective team for the larger business in a lot of ways.