In a legal context, hacking is a term for utilizing an unconventional or illicit means to gain unauthorized access to a digital device, computer system, or network.

We can rule out "illicit" because the FBI published the data publicly. Now the heavy lifting has to be done by "unconventional", which I don't think qualifies here. A government agency published the credentials, which means no one had to do social engineering, sneak into an office, reverse engineer anything, or even guess a person's birthday.

Now if this somehow went to court, a judge might rule that this qualifies as hacking, but my opinion is that it doesn't.