Does this mean "circus performers"? I've never heard the phrase circus artists before.
In Britain, we call circus performers "circus artistes" not "circus artists".
Painters, sculptors, and maybe musicians are artists. Musicians, dancers, and gymnasts are artistes.
Performers should be accepted. I've never heard the phrase "circus artists" in the U.S.
Why de and not du?
because it is a generality, it is not about this circus (and we would say : de ce cirque) but about the job.
With "Les artistes DU cirque" is is about a specific circus.
It offers "clowns" as a possible synonym for "artistes" and then rejects "These circus clowns are really terrific."
"These circus performers are truly incredible." Should be accepted
'... really wonderful' should be accepted
Performers should be accepted here. Duo is taking the translation literally and isn't considering popular American or British English.