Difford has halved the syrups from the original recipe but only reduced the citrus by 5ml each, so that may be your problem. I tried his version and found it very heavy on the Ango, so I might try adding a little more syrup too.
Partly because recipes vary between 1:1 and 2:1, I tend to operate from a 1.5:1 baseline. Personally, I don't notice perceived sweetness scaling directly with said ratio, but it would still undoubtedly play a role.