Why does it need to be a subset? Then you run the risk that whoever uncovers wrongdoing is labeled "not a journalist". Same reason freedom of speech applies to everybody and not just registered "free speakers".
You might argue that we shouldn't have secrets at all and everything should be public.
I think that's a worthy goal, but doesn't work in practice.
If another state is competitive with you (and all states are competitive with each other to some extent), then I think you need the ability to keep some information confidential somehow.
Responsible disclosure of "secrets" in service of public interest is, to me, one of the defining characteristic of a free and thoughtful society.
[p.s.] It should be noted that your position is effectively arguing that 'Journalism is a component of state-craft and as such can only be practiced by official members of the governing class'.
Perhaps I’m blinkered, but I can’t see how a modern state can function without them in some form. To offer an extreme example, nuclear launch codes must remain secret. (Separate argument obvs about the merits for and against nuclear weapons).
If the state is going to have some secrets, I don’t see how they can work in practice if everyone is allowed to know them.
Separately I’m not sure I buy the slippery slope argument as an absolute given. It’s a clear risk yes, but I see good journalism as one of the safeguards against all forms of government malpractice, corruption or straight incompetence.
[It should be also noted that the fact that state craft requires secrets is not a carte blanche to the state to commit illegal acts.]
Nuclear launch codes can remain secret. It is difficult to see how the disclosure could possibly be both "respnsible" and "in the public interest".
Disclosing that certain government agencies are running projects like "MK-Ultra" [1], "Mockingbird" [2], "Artichoke" [3] is in the public interest.
(You know, one of the characters in one of John le Carré's Smiley books once said: "A nation's secret services are the true reflection of its soul"...)
[1]: https://en.wikipedia.org/wiki/Project_MKUltra
An analogue would be arguing that you have to be an official whistleblower in order to be protected by whistleblower laws, rather than simply being any citizen who meets the criteria necessary to be protected. It's just not a tenable way to serve the public good.