I think the Front Plate is Carbon Fiber. at least it looks like it from the 2018 catalog for HO rear
The Campy Technical Document lists the HO 11 spd 2018 Rear Der as
RECORDTM REAR DERAILLEUR
New upper body with HO (Hydraulic Optimization) geometry in Ultra-light technopolymer reinforced with carbon
fibre - Upper pulley wheels with longer teeth - Lower pulley wheels with rounded teeth - Embrace TechnologyTM
- Carbon fiber cage construction - Front plate in carbon fibre - lower body in monolithic technopolymer with carbon
elements - Aluminum fixing bolt
AS you mentioned, I have the 2018 Record 11spd HO medium cage and I do believe it's aluminum. Here's the medium cage description which clearly identifies the "aluminum cage". I run Medium 2018 Record HO & Chorus HO on 2 bikes.
RECORDTM REAR DER. - MEDIUM
72,5 mm aluminum cage to fit the 11-32 Campagnolo 11 cassette - New upper body HO (Hydraulic Optimization)
geometry in Ultra-light technopolymer reinforced with carbon fibre - Travel limit screws positioned on the rear
of the upper body - Upper pulley wheels with longer teeth - Lower pulley wheels with rounded teeth - Embrace
TechnologyTM - Front plate in carbon fibre - lower body in monolithic technopolymer with carbon elements - Aluminum
fixing bolt