If I were to guess, the fibers are used for the actual high bandwidth stuff, and the wires are used for the low bandwidth stuff (auxiliary, network etc) and to transfer a bit of power to the other end to power the converter that converts the data on fiber back to regular wires ... just in case the remote device does not provide enough power or no power at all.
See pinout on the side of this page :
https://en.wikipedia.org/wiki/HDMIPin 1 - 12 : 3 TMDS pairs and their shielding.
Pin 13 CEC
Pin 14
Reserved (HDMI 1.0–1.3a)
Utility/HEAC+ (HDMI 1.4+, optional, HDMI Ethernet Channel and Audio Return Channel)
Pin 15 SCL (I²C serial clock for DDC)
Pin 16 SDA (I²C serial data for DDC)
Pin 17 Ground (for DDC, CEC, ARC, and HEC)
Pin 18 +5 V (min. 0.055 A)[3]
Pin 19
Hot Plug Detect (all versions)
HEAC− (HDMI 1.4+, optional, HDMI Ethernet Channel and Audio Return Channel)