Okay, let me try to explain this one more time.
In that same trailer, Samus changes into ZSS after she shoots a big beam. By your logic, this confirms that this is how it will work in gameplay. By my logic, that is a possibility, but we can't know for sure how the characters will work yet.
That's why when someone says, "Samus transforms into Zamus after shooting her Final Smash Blaster Beam!" I respond by saying, "That is one possibility, but ultimately we do not know for sure yet! That sequence might have been just a cinematic to introduce the character. We'll have to wait and see."
Then, by your logic, if Sakurai confirmed Monday that Samus transforms into ZSS after shooting her Final Smash, it would be my turn to eat crow...?
See?
If you're not getting it now, you never will.