It looks like
EditString::SetData and EditString::SetText operate to different values while working with SetData/GetData is a common way of operating internal control value for all other controls. I believe EditString::SetData should set control's text.
It does, indirectly. (If not, provide an example please).
As for SetData and SetText differences, they are trivial with EditString, but important for e.g. EditInt...