If there is more than one type of sugar and the amount of each type has been halved then I think it makes sense to say 50% less sugars. But I don't know if that's what they intend.
Suppose it has 10g of sucrose and 10g of fructose, so it contains "sugars". "50% less sugars" would mean it's reduced to 5g of sucrose and 5g of fructose. "50% fewer sugars" would mean it's reduced to only having sucrose (say), ie the fructose has been eliminated.
However, I sympathise when you say it feels wrong: I think the pedantic ear is programmed to react against a combination of LESS plus a plural noun ending in S, even when it is in fact a valid construction.