It does mean what you suggest, but if you go in a group and defer you could be split around the stadium when you come back, same if one defers in a good level 1 or 2 seat and only gets the option of 3rd tier.I assume the guarantee of keeping your seat is keeping the same seat not a seat. Which I get as the relocatipn window might not block our individual seats. Maybe it should but that would be my guess at the wording.
Also it could be a more expensive seat you are offered, and it is out of the question to take the gamble for most people.