I'll have to check, but I don't think the screen has an HDMI in...
Edit - Aye, this is from the specs as listed on pimoroni:
Connects to the Raspberry Pi board using a ribbon cable connected to the DSI port.
S'gotta be that the HDMI won't pass the touchscreen commands back to the Pi.
EDITED: 14 Sep 2018 15:45 by GRAPHITONE