Respectfully mention that it's kinda insensitive, kinda like, "hey man, don't you think that's kinda insensitive?" and you'll probably figure out fairly quickly which one is the answer. Additionally, if they legit don't know why it's insensitive, then you can educate them!

Just remember that tone can be difficult to convey on the internet, and sometimes you have to exaggerate the intended tone for it to be understood. Additionally, in my experience, asking the question from the other person's perspective ("don't you think that's kinda insensitive" vs "I think that's kinda insensitive") seems to help a lot.