The article explicitly mentions predictable work.

And says that's what you pay hourly for. Says that in detail. Misunderstands that the experts are generally hired to "discover" or "invent" an answer that wasn't known, and discovery / invention often isn't estimable.

Further, author goes into risk, the real risk is of course acceptability of the expert's answer, which involves the buyer "being brought along" with accepting the answer, a process the expert genuinely cannot predict.