No, it is not. It is public information. The only thing kid did is tweet information that can be accessed by all persons in the US. Do you mind not being stubborn on things you don't know about?
Is a Twitter account that uses this API to post all movements of a single private individual, or their vehicle really so unobjectionable, just because it comes from a public API?
At some point we will need either very strong social norms, or case law, or most likely legislation to address this issue.
Addressing the privacy impacts of programmatic operations on public data (i.e. what you can see from a window, or a satellite) is an important frontier for privacy and a largely unsettled question. You're just seeing it play out in this case because data for this particular vehicle type is public.
(Which, I should add, is surely just for legacy reasons and definitely a terrible idea. There's no public interest in being able to track everyone with a plane any more than there is a public interest in being able to see comparable data for anyone with a bicycle, car, or cellphone. Or look at it this way: when private flight become 100x cheaper and safer to the point where we fly instead of drive, do we want our movements tracked just because we were in the air? Of course not.)
I doubt private flying will become that frequent within the next 100 years. How would it possibly be safer than driving? Now instead of just dealing with distracted, sleepy, drunk drivers on the road, I have to worry about accidents raining down from the sky? Please, just build train systems and well designed, well-zoned, walkable cities and we won’t need to worry about flying.
I'm on the board of an organization (fightforthefuture.org) that has been leading on passing legislation for limiting the use of biometric surveillance, like face recognition. That's a small subfield of the larger problem of the privacy impacts of programatically analyzed public data, but people seem to understand it intuitively and that makes it a good place to start.
Also, I mentioned the flying cars example not to assert that there will be flying cars, but as a way to illustrate why this data should not be public: i.e. because it would be clearly bad to have this data public for a more popular vehicle type.
I should also say that it is definitely wrong for people with Musk's level of resources to sue people without those resources over something probably this minor.
I'm just chiming in to point out that we should not consider aggregated public data fair game just because it comes from public data, because some uses of this data (including this one, though again probably in a minor way, though I say "minor" knowing almost nothing about Elon Musk's personal threat model so maybe I'm wrong) can have a harmful impact on individuals and the public.