Not unless she lies to herself as convincingly as she lies to the public.
Nyad researched the swim before she completed it in 1975. In her 1978 memoir, Other Shores, she documents her findings:
Ida Elionsky’s time was a very respectable 11 hours, 35 minutes. There were others who made the swim…but I will simply mention the male and female record holders. Diane Struble plunged into the Battery waters on August 15, 1959, and reached her original starting point in 11 hours, 21 minutes. (p. 57)
So Nyad knew the names and details of at least two and probably more of her predecessors. By 1981, though, she began claiming to be first, and she never stopped—except for a few months after she got caught.
But just as with the Cuba-Florida swim, Nyad was desperate to be first. So less than a year later, she stole back the title from Ida Elionsky. In a May 2012 Elle profile, Rivka Galchen reports that no women had swum Manhattan before Nyad.
A few months later, Nyad declares on camera that she was “the first woman to swim around Manhattan Island, you know, and that sort of thing” (Diana: A Documentary, 4:00). She devotes an entire chapter of her 2015 memoir, Find a Way, to her Manhattan first.
Nyad’s second memoir, Find a Way, appears. She devotes a chapter to her Manhattan first.
Toward the end of “History Rewritten…,” Nyad proclaims, “I hereby relinquish my title as the first woman.” But she had nothing to relinquish. She lied about being first, then lied about lying about it. “I take both joy and pride,” she continues, “in honoring all great women athletes of all eras.” Trying to erase her predecessors from history, however, sounds more like malice and envy than respect.
Stealing first: In footage from multiple sources, Diana Nyad claims to be the first woman to circle Manhattan (“and that sort of thing”).