I've never before come across that "Zulu" story as far as I can recall. What I've heard was that Sulu was named in honor of Herb Solow (pronounced "solo"), the Desilu programming exec who bought Star Trek
and oversaw its production. But I have heard that it was also derived from the name of the Sulu Sea.
And yes, Sulu was definitely intended to be pan-Asian. 1968's The Making of Star Trek
says he is "of mixed Oriental and Filipino background" -- which further reinforces the idea that he was named for the sea in the Philippines -- although it also says his "cultural heritage is mainly Japanese." It seems to me that the character was conceived as generically Asian, but once they cast a Japanese-American actor in the role, that influenced writers to treat the character as Japanese.
(The name Hikaru was coined in the tie-in novel The Entropy Effect
by Vonda N. McIntyre in 1981. It was finally made canonical in The Undiscovered Country
a decade later.)