Ascorbic acid is not a sequestrant, it is a stain remover. Stain removers lift stains off of surfaces. Sequestrants bind to metals in the water and prevent them from forming stains. Sequestrant will occasionally help remove a stain from a surface, but that is not it's primary purpose.
Ascorbic acid has the fewest "side issues" of any of the stain removers. If it works it is either the same or simpler to use that anything else.
Removing copper stains in particular is unpredictable. Sometimes they come off fairly easily, other times they can be very difficult to remove. Copper can take several different forms, each of which responds differently to stain removers.