People found out the Reddit was his because it was the profile that he used for doing a Q&A for a show he was in a while back. Apparently there was even a post where he says his name. So once people found this profile and knew it was attributed to him, they basically just went back on his post history and found everything. It's actually not that hard, specially when people have a lot of time on their hands.
I definitely could see that happening, although I do wonder if they would give him the traditional 10 Rings, which are so different in the comics, or they would find a way to give him the ''arm rings'' from the movies and basically sell it as brand new artifacts on Marvel Comics lore. I feel like the second option might be better, 'cause then Shang could have his own mythology instead of feeling like he is ripping off the Mandarin stuff. I'm not even sure if connecting him to the Mandarin in the comics is a good idea since the MCU actually worked hard to erase that connection and show his father is not ''the Mandarin'' (he even mocks the name in the movie).