But for users without the rep, is the 'flag' function a viable means to recommend to a Moderator that a question should be protected, or is this not the right method?
You can also drop a line in chat: there there are frequently a couple of high-rep users hanging around who can protect questions (though 15k+ users should wait a bit more than mods to protect a question).
However, I'm not sure I'd protect that question right now, the situation doesn't seem so bad to warrant protection.
Short answer: Yes.
However, questions are generally protected only when they are attracting large numbers of junk answers or comments or other unwanted behavior by low rep users is taking place.