Saying X defines a sandbox, is a fallacy, if it also applies to other styles.
Unless X is merely part of the definition. If you say I can't define American football in terms of balls or players because other sports also have balls or players, then we would be at something of an impasse.
If I were to look at defining a sandbox, I would say this:
(1) First, you need to understand the distinction between prepping a plot and
prepping a scenario.
(2) A sandbox is what happens when you apply the same design aesthetic to the entire structure of the campaign.
The easiest way to understand this design aesthetic, IMO, is through a negative definition: A sandbox campaign is one in which the GM does not negate choices made by the players in order to enforce a preconceived path.
I think there are ways to:
(a) Prep the campaign to make it less likely that the GM will be tempted to enforce a preconceived path.
(b) Prep the campaign to make it easier to run the sandbox.
(c) Prep the campaign to make the sandbox richer and more enjoyable.
(d) Use a sandbox to achieve specific goals which are possible and/or easier in a sandbox which would be impossible and/or harder to achieve outside of a sandbox.
But I also think it's quite possible to sit down with absolutely no prep whatsoever and run a sandbox campaign. I think it's even possible to sit down with a published module designed to encourage and/or require horrible railroading and
still run it as part of a sandbox (by ignoring those portions of the module that require railroading, even if it means much of the rest of the module becomes useless).
If the GM has already decided there are no dragons in the area, but retcons in a dragon lair, where before there was none, as a response to player request, that seems to me to be an approach to world building which is against the sandbox ethos. If it is more fun for all concerned then it's not a problem, though. It's not inherently badwrongfun.
IMO, there's no meaningful difference between "I want to play in a campaign where I fight dragons" and "I want to play in a campaign where I'm a fantasy hero". This would be particularly true if both statements are made before the campaign even begins; but remains true, IMO, even if those statements are made after the campaign has begun.
The degree to which either desire can be successfully catered to depends on a number of factors (including the fun of the other players and the GM), but the idea that a Traveller sandbox stops being a Traveller sandbox because somebody said "I want to pilot a spaceship" seems irrational to me.