Is it possible that Sian is thinking of the aria from Handel's Rinaldo instead? From an article on Handel on the NNDB site:

"It [Almira] contains many themes used by Handel in well-known later works; but the current statement that the famous aria in Rinaldo, "Lascia ch'io pianga", comes from a saraband in Almira, is based upon nothing more definite than the inevitable resemblance between the simplest possible forms of saraband-rhythm."

And from the Online Encyclopedia:

"All Handel's early works contain material that he used often with very little alteration later on, and, though the famous " Lascia ch'io pianga " does not occur in Almira, it occurs note for note in Agrippina and the two Italian oratorios."

While I suspect Fuld's assertion is spurious, I haven't the resources to either prove or disprove it. To start, there is clearly no resemblance between "Juanita" and the Rinaldo aria, and, from what I can tell, the Almira sarabandes are untitled. There are two sarabandes I'm aware of, in Act 1 Scene 1 and Act 3 Scene 2, but I haven't found a source that clarifies which of them was purportedly recycled for "Lascia". The sound clips for Almira that I've found don't include the sarabande portions of the tracks. I found one sarabande as a separate track--it wasn't remotely like either "Juanita" or "Lascia"--, but the listing neglected to indicate which part of the opera it was from.

Side note: The "Lascia" from Rinaldo was covered by the Celtic Woman ensemble (it's on the "A New Journey" album), and also by Sarah Brightman. Y'all may know it without knowing you know it.