I certainly don't think she's ducking her, but these circumstances are exactly the circumstances in which an athlete would duck someone else.
The favorite who always wins is under more pressure to continue to win than the athlete who doesn't win very often. In this scenario, Sydney has everything to lose and little to gain and Bol the exact opposite.
Also, moving down to a distance that everyone knows is not your strength also takes the pressure off.
But that said, as was mentioned, this is probably a sponsor thing. Pretty unlikely she'll be 'ducking' Bol in a 500.