Lead(II) acetate (Pb(CH3COO)2), also known as lead acetate, lead diacetate, plumbous acetate, sugar of lead, lead sugar, salt of Saturn, or Goulard's powder, is a white crystalline chemical compound with a sweet taste.
So clearly (?) it would add a sweet taste, regardless of how much of the acid was chemically consumed.
I'm sure it started accidentally but was continued because it magically turned shitty sour wine into sweet wine.
A side note; we still actively use artificial sweeteners from petrochemical sources.