All the ground cables have more DC and RF resistance then the aluminum plate!
As a result they look impressive but mean nothing.
The board has 4 mounting holes each is ground so if mounted to a metallic plate
with metal spacers and screws the board is grounded. Anything more than
maybe using star washers is pointless.
The use of a metallic box is however a good thing and insure there are grill or
holes for airflow as the finals do require airflow.
Using insulated mounting kits for the finals and mounting them to a grounded
heat sink of some larger radiating area is also a good procedure.