the "official, authorized" Yma Sumac site, has the dope on this legend. I don't know where you were looking Steve the elf, but here you go:
"On the way to New York's Roxy Theater on Broadway, one cold February evening in 1951, two musicians were walking along with Hernán Braña, Yma's flautist, drummer, and good friend . Yma was appearing on the same bill as Danny Kaye. Seeing YMA SUMAC in huge letters on the brightly lighted marquee overlooking "The Great White Way," one of the musicians laughingly turned to Hernán and the other musician and said: "Hey look, it's Amy Camus!" They all had a good laugh and repeated the story to the orchestra.
The story eventually reached Walter Winchell who stuck it in his column of usual vile slime. The entire story was made up for laughs. Amy Camus, the Jewish housewife from Brooklyn, was born in 1951 at the Roxy by a musician from Brooklyn having fun with Yma's unusual name. Yma thought the story was really funny and for publicity purposes, she subsequently accepted a presentation from the borough president as "Honorary Citizen of Brooklyn" in 1957. Anyone who knows or has heard Yma speak English recognizes that her charming accent is pure Peruvian, not anything resembling Brooklynese. "