I would expect that it is there due to its poor physical bulk, but it also has 124 Special Attack, high Special bulk, and speed. I'm not sure why it's there, because the upsides clearly outweigh the downsides.
However, Gardevoir is rather slow for a Choice Scarf user, so it can't revenge kill most other Choice Scarf users in the tier such as Heracross, which can easily pick it off with minimal chip damage.
All in all, quite the special wallbreaker/Trick set but only under very specific conditions. Gardevoir only exists on RU because there are much better versions of itself in every other format, so it just has no usage. Hope I helped! Smogon‘s Gardevoir Page