Quite simply, the dictionary definitions do not apply in this case. There is a common community definition that is being used, not the dictionary definition.
The community definition is such:
A post-transition phase where one has reached a point where they are no living as their birth sex, and that people in their lives are unaware of said persons birth sex.
Hopefully that clarifies things. If you have not transitioned, you are by definition not stealth...you are in the closet, and there is a big difference. One transition is over and done with, the other transition has yet to take place. One cannot go stealth, while living as their birth sex, because one has not yet transitioned.
Stealth and in-the-closet are two different places. One transition is done with, the other transition has not even started. These are commonly used definitions within the community...and yes, its pretty clear, you have much to learn.