Position in the production hierarchy.
Consider a person that carves bespoke wooden figurines. Their job takes considerable skill, even talent. But it takes then X hours to make Y figurines. Time in, widgets out, completely proportional.
Now imagine a person that configures a robot to carve crappy mass produced figurines. Their work is differently skilled, but perhaps less so. The product isn't as good. Yet they put in their time once and the machine cranks out enormous quantities with minimal additional work from them. This has high leverage, they can command more pay because their leveraged work has more value.
You can go further up the hierarchy, to the people who organize or direct the technicians or the channels that distribute the products of their machines. Their work is further abstracted, skilled in its own way but not necessarily more so. But the amount of income their efforts command is greater still-- even more leveraged.
In areas where the use of the high-skill low-leverage work are largely optional-- say custom carpentry-- they often become unavailable because the sort of highly competent people who would excel at these tasks can make more money in a highly leveraged (but perhaps less skilled!) job writing boilerplate javascript to deliver advertising or whatever. In areas where the non-scalable work isn't optional, like medical doctors-- the prices tend to shoot through the sky in part due to the income an equivalent aptitude could command in a more leveraged position.
In between the two extremes we have workers that are just flat underpaid, and we rationalize the situation by imagining that they're less deserving.
As the article points out, many of these lowly paid jobs are highly skilled in their own way-- at least if done well. What they don't have, however, is a big initial training barrier to entry that limits the supply of capable people and puts market power for this labor onto the supply side.
Skill isn't a factor in wages except in so far as the required skill impacts the level of supply or demand in the supply/demand balance.
I think we often underestimate the capability of the average person because we can't know the average person and can't see first hand the depth of their ability and because we also so often put that average person in a situation that just doesn't exercise those latent abilities (or do, but do so in ways we don't respect due to cultural biases).