Here's the thing.
Tsunku's comment says:
Early in May this year
“Earlier, I have been contemplating and consolidating with staffs, about returning to being a normal middle school student. I feel very strongly about this, please let me quit.”
she has made this request.
...
It was officially announced at first that this unit’s member might increase or decrease,
it was due this is reason that an urgent S/mileage member audition was held in June this year.
Due to various conditions of the group, herself and other parties involved, as well as terms and conditions on contract, it was not until today that this information about her could be reported, please accept my apologizes.
(emphasis added)
Now if any of that's true, the audition was decided as a direct result of Saki's decision, not the other way around.
Keep in mind that the new member audition announcement came at the
end of May, not the beginning.
While it's true that you could interpret her lack of enthusiasm throughout the entire process as being bitter about having new members added,
you could just as easily say she was made saddened by the fact that adding new members is a concrete sign that the group will go on without her.
It's like "You really
are going on without me, aren't you?".
If anything, that just reinforces the feelings behind her decision to leave.
It might just be spin, but the truth is we have no idea what really happens behind the scenes.
Unless there's some outside source that we have, we can only take these announcements as they come.