I don't want to involve myself in this argument but I will say that I can not see their original post. It displays as just "[flagged]" for me. I don't know if that's the app I'm using or what, but I cannot see the post.

Really? I see the "[flagged]", and the post, it's just slightly dimmer and I had to manually open it. I'm using no special extensions or anything, just a normal browser.

There is a stage past [flagged] where a post can also be [dead].

You only see dead posts if you have showdead=yes in your settings.

So you may have seen their comment before it was killed, or you may have showdead=yes in your settings.