I actually don’t think this is correct. Whom is used when the unknown person is the object being impacted (to whom did you sell your car). In this sentence, “who” is actually referring to a person performing the action (the sentences “who questioned him?” And “where are those who questioned him?” Would use who, not whom. You wouldn’t say “whom questioned him,” but whom could be used to replace “him,” such as “he questioned whom?”).
As I alluded to above, you can usually see if it’s who or whom by changing it to he/she (who) or him/her (whom). You may need to adjust the sentence slightly, but it will normally work. Above you need to remove the “where are those” portion to find the answer.
So I believe that your correction came across a bit rude, and I’m fairly certain it is also wrong.
I appreciate your reply and question. I think it comes across rude because the correction wasn’t really necessary to understand the context of what was said, and it’s even less of a correction and more of a personal preference. It doesn’t add much to the conversation, which makes it seem more like grammatical pedantry. The fact that it wasn’t entirely correct made it seem even more out of place.
For the record, I’m not accusing you of being rude, just identifying how the comment came across. I’m not assuming intent with your original comment and apologize if it seemed like I had.