Some of the answers to this question are reacting to a value judgement that isn't necessarily present in the term.

A 'knowledge worker' is someone who primarily handles information in their work, in contrast to someone who primarily handles physical materials. There's no judgement being made on skill or value, even though the original author's "other" type of worker was referred to as 'manual' which sounds somewhat judgemental.

A case could be made for some doctors -- surgeons for example -- being classed as 'manual' workers.

Some of the answers to this question are reacting to a value judgement that isn't necessarily present in the term.

A knowledge worker is someone who primarily handles information in their work, in contrast to someone who primarily handles physical materials. There's no judgement being made on skill or value, even though the original author's "other" type of worker was referred to as a manual worker which sounds somewhat judgemental.

They are knowledge workers not because they need knowledge to do their work, but because their work is to operate on knowledge, transforming it etc. It is the material on which they work that they are named for.

A case could be made for some doctors — surgeons for example — being classed as 'manual' workers.

(edited to add third paragraph, clarifying distinction between need / operate on)