That video is probably fake or doctored. Many have sensed the warming of the iphone 7 a bit when charging, but far below temperature of ignition.
In addition to leaving individual feedback for Apple, we should try to compare expieriences here to try to isolate the precise factors that cause it to heat. After all it is in our best interest.
MY HEATING FACTORS
• My iPhone 7-128GB does NOT seem to heat when not charging.
• It also does not seem to heat when charging while using apps like Mail, iMessage, Phone, Pages, Word, etc.
• The heating seems to occur while charging, if I use a video app, particularly YouTube, whether viewing a video on the phone or mirroring to a TV.
• It cools down while charging when I close the video app and put the iPhone to sleep for a few minutes.