Your point that there are hierarchies within the skill set of various tasks is true, however the argument isn't that everyone needs to be a KE worker - it's that everyone needs the opportunity to be one. Presently that is not the case, many are completely shutout simply because of birth lottery.

A core part of Unger's argument is the expansion of a vital suite of protections for all people. E.g. if everyone has access to universal healthcare, greatly expanding public housing verticals, ample opportunities to retrain and direct their lives it fundamentally changes the nature of our relationship to work.

The grocery store job is only dead-end now because our well being is entirely dependent on our ability to generate capital. In this reimagined future it might be the perfect job for a new mom (or dad) who wants to focus on spending as much time as possible with their child while still having some human interaction outside the house.

If everyone is capable of being a KE worker, it doesn't mean they will be. When and if they want to they can.