No, I think you've got it right. Adding a modifier to the die before you test it for success appears to bork the dice string. When I tested it for writing the guide, I must have only tested it for dice pools (i.e. for counting success in 6d10 for White Wolf, et al.).
@Eric, is this intended behavior?
[Edit] - for clarity - success(1d20+5, 16, above) does not work but success(1d20, 16, above) does.