Because of the order of parameters in the call to extend, the schema defaults override the event's values.
The normal meaning of default would imply the reverse.
I don't know if this can be changed at this point. If not, the docs should warn you about it.
Version: unspecified
Severity: normal
First of all: wow. Good catch. I don't know how this slipped past -- it is egregious.
It looks like we may be saved by the fact that setDefaults was not advertised well and thus not used outside of E3: GuidedTour and GettingStarted are the only instances of setDefault use in the wild that I have been able to identify.
GuidedTour is okay, since it only uses it for userId and it never overrides that (nor would it make sense too).
Related URL: https://gerrit.wikimedia.org/r/62793 (Gerrit Change Id14ba276329272c41370458d5330599a725f5419)