I doubt adding sugar will change the FG. I believe it will ferment out resulting in higher ABV but will take you right back to the same FG when it’s done.
I think there are two parts to the theory behind adding some sugar. The first is that the volume of the batch is being increased (albeit a very small amount) with something that is highly fermentable. So the main batch will have a certain FG, plus the sugar addition with a potentially much lower FG, together will result in a lower FG for the entire batch. The other thing is with the second sugar addition the yeast will grow and become active again and could possibly pickup some additional existing sugars that were already there. I don't recall where I saw the second one. Not sure if that happens or not.
I have see this work in practice though. I brewed a honey wheat ale once. The primary fermentation stop at 1.008. I added honey which brought the SG back up to 1.016. The fermentation picked back up and the FG ended up being 1.006 after the secondary.
If there is a change in FG though, I would expect it to be small.