You're suggesting that in order to study the history of something, you also need to be a practitioner of it? So for example, a professor of military history needs to have combat experience? An art history teacher needs to be an artist?
I don't know if the parent is suggesting it, but I surely am. You can do it without being a practitioner, but worse (everything else, like your knowledge of the subject or writing skills, being equal).
Practitioner doesn't mean "illegal practictioner".
Arson investigators' training (and even practice) includes setting up controlled fires to study how arsonists works or how a specific arson case played out...
Does anyone seriously has to put this lame "so to be a professor/investigator of X, one must do X?" (illegal or impossible) question, when the answer is not only obvious ("no, but they should be aquainted with X to the best possible and legal degree") but has been clarified again and again in this thread?
Your quoted sentence does not imply or suggest this.
An art history teacher would be well served having tried her hands at using the historical techniques a few times to get a feel for them. But no need for her to be a brilliant artist.
Lecturing on music without being able to play music is also suspicious.
I would feel very fake if I were one of those people.
You would only be able to use second hand sources where someone with real ability has predigested the original works for you.
A psychologist, who learned everything about psychology, without ever having actually practised it, is equally skilled as someone with experience. He isn't.
And the comparison does not work even in context of original topic. The article from Beard linked from this blogpost (or blog post itself).
For example, if you want to write the work of a mathematician but cant read or understand the writings of the mathematician, then I would be uncomfortable with that.
All other things being equal first hand practical knowledge is then an edge, so not required but a benefit.
In that they should be able to read the ancient languages of the culture they study, for one.
Nobody said anything about "firsthand knowledge" in the sense of "having lived in the time of the ancients".
But if you insist, sure, a historian that have actually lived at the time, would have a far better understanding of it (all other things being equal, like writing ability and so on). I'd take Thucylides over any modern historian of the Peloponesian war, for example.