"Is there doubt about how experienced he is?"
Translation:Ĉu estas dubo pri kiel sperta li estas?
Note that in this course, the Esperanto sentences are primary and the English is just translation. It probably doesn't pay to speculate too hard on alternate backward translations into Esperanto.
That said, the task here (in the exercise implied by the sentence pair in the OP at the top of this thread) is to translate the expression "how experienced." Kiel is the correct word here. Think about possible answers to the question "how experienced?"
- very experienced - tre sperta.
- somewhat experienced - iom sperta.
- slightly experienced - iomete sperta.
Very, somewhat, slightly, tre, iom, and iomete are all adverbs of degree, not quantity -- so we use kiel. Zubiz is the only one in this thread who proposed an actual sentence for consideration:
- "ĉu estas dubo pri kiom da sperto li havas?"
I agree that this is a valid sentence in Esperanto - but it means something different: Is there a doubt about how much experience he has.
Here are the links to this thread, and to the reverse thread in case anybody would like to see more discussion about this sentence.